High Sierra Compatibility with Mac Pro 5.1

You can always install High Sierra on a drive other than your main system drive: you have room for 4 drives in your beautiful silver tower. You could also apply the upgrade to a clone of your current main system drive (SuperDuper or Carbon Copy Cloner can be used to clone your drive) to see if you are going to have any problems with it.

If you are using a solid state drive (SSD) in addition to spinning hard drives, you may not want to apply the upgrade, due to the file system change introduced with High Sierra. Also, High Sierra may very well require a firmware update to your Mac Pro, which might be yet another reason for caution.

I have a 2010 Mac Pro also, and have no plans to install High Sierra. For my purposes, there is no benefit to doing so.

We have nearly the same machine. Mine is the 3.33 GHz model and has 16 GB of RAM installed. I put an SSD in it almost three years ago.

High Sierra is running without issue for me. Even the .0 release was pretty good. That said, I never jump straight in. This is my production machine. I split my SSD into two partitions and installed High Sierra onto the blank partition. Then installed all of my apps fresh. That allows me to install things as I have time, and can still boot to my previous OS to get work done.

With the release of 10.13.1, I couldn't see any reason not to move to High Sierra as my every day OS.

Like my Mac Pro, yours will likely require a firmware update. It will do that before installing HS. It makes no difference the rest of your installed drives are standard rotational models. That's how my Mac Pro is setup. The OS and all apps are on the SSD. The other drives are for everything else. Like images, video and other working files.

The firmware update is necessary so the Mac can recognize an APFS drive, which the partition on any SSD is converted to when High Sierra is installed. You don't get a choice. SSD = APFS. Without the update, the Mac will not be able to boot to an APFS drive, or even have one appear on the desktop, in Disk Utility, or anywhere else.

As with any firmware update, you must ensure the device cannot lose power during the operation. In this case, borked firmware - Mac no boot.

I have pretty much all of our equipment on UPS devices (Uninterruptible Power Supply). You should, anyway. They protect the equipment against power surges, and even near lightning strikes. Each UPS comes with a large warranty replacement guarantee. Any device that gets still gets zapped behind the UPS is covered by the company. APC (American Power Conversions) is probably the best known brand.

They're also a huge benefit in brownouts or blackouts. Power goes out, the computer keeps running on the UPS' internal battery. How long depends on how much AC power it has to produce to keep the device running. But it's always long enough to save your data and do a proper shutdown instead of having your computer just drop.

All apps I use continue to work as expected. And I jumped from El Capitan to High Sierra. I kind of expected a bit of a slow down, but High Sierra turned out to be faster than El Capitan. That may partially be due to the fact that APFS is a faster, and much more robust file system than HFS+. I also much prefer a clean system. I never install an OS upgrade over a previous one. I have to presume that's why I never see the many problems other users report when they upgrade. Yes, it's a pain to load many large apps from scratch (the Adobe CC suite, Quark XPress, i1Profiler, MS Office and many others), but in my opinion, worth the effort.

As of 10.3.4, you'll see a warning pop up for 32 bit apps (only the first time such an app is run) that the app needs to be updated. That's not quite it, but I can't recall now how the message is worded. Nothing is really wrong. Your 32 bit apps will still run without issue. What it's saying is in the next major release of macOS, Apple is going to start pulling 32 bit support out of the OS. A 32 bit app may run in the next OS, or it may not. Or will run, but not well. The OS after that, 32 bit support will be completely gone. 64 bit apps only.

If you keep a dual boot setup, such as High Sierra on one partition and El Capitan on another, when you boot into El Capitan or older, APFS drives will not show up at all. The older OS's have no clue what APFS is. Sierra might, since users could kind of beta test APFS live if they wanted to. But I haven't tested that one.

Oh, and you don't need to convert your other drives. They can all stay as HFS+ if you want. I even have High Sierra on a rotational drive formatted as HFS+.

Almost every OS upgrade gets a couple of articles about the "disaster" usually including the thought that while Jobs was in control this would never have happened. Of course some of those "disasters" occurred under his watch. The truth is more complex. First, all software ships with bugs and the MacOS is no different. The intent however is that the known bugs are minor annoyances and not full blown disasters but even so the cautious consumer lets others be the first adopters. I like to be on the bleeding edge but I always have one computer or one boot drive dedicated to the previous OS just in case. My habit extends to updates as well - before updating I always make sure my alternative boot drive is up to date.

Second, lots of people jump into an upgrade with a computer that isn't ready to be upgraded. They might have long standing problems that they hope the new OS will fix. It probably won't. It may make the problem worse. They may not have bothered to ensure that all their software is compatible with the new upgrade. Sadly, sometimes the upgrade won't be supported by peripheral manufacturers so a new scanner or printer might be required. Apple will get blamed for these self inflicted problems too.

Is High Sierra perfect? Nope and it could be argued that its roll out was a bit more rocky than some recent upgrades. A lot of under the hood changes were made. But I've been using it trouble free since its release...then again these days my needs are pretty vanilla.

I was running it on 2011 MacBook Air with 4GB and saw little to no performance hit from Sierra but I did notice the MBA was happier when I ran fewer apps. High Sierra is more RAM hungry. I'd definitely not want to run it on a system without an SSD

have not had any problems with my imac 2011

macOS High Sierra - Technical Specifications

but from peoples reports it's pretty slow if one only has 4GB of system mem

Please read Upgrading macOS without fear.