Will I see speed increase with thunderbolt/Ether adapter?

That adapter requires that your computer has got a Thunderbolt port. Unfortunately, the Mac Pro hasn't got a Thunderbolt port. The first Mac Pro with Thunderbolt will be launched this fall.

In this case, that adapter won't work. Also, it shouldn't improve your performance

Your current Ethernet port is capable of running as fast as the device acting as the Ethernet Switch on your network. In most cases, this is an Internet Router. 100BaseT is a typical speed for these devices. This is three times faster than the bandwidth of the Internet connection your ISP is providing, so it will not be a bottleneck to performance. Raising that to a faster speed will also not be a bottleneck to Internet performance.

You can check the actual connected speed in Network Utility, in the Info pane. (Network Utility is in the /Applications/Utilities folder.)

If you use an Ethernet Switch capable of Gigabit Ethernet speeds, as I do, it will allow faster File Sharing for files that are strictly local. The Internet Router does not intervene in local transfers, so they move very quickly between my Home Server and each of the computers. When data is addressed to the Internet, it goes down the comparatively-slower 100Base-T cable to the Router, and then on to the Internet.

Note that if you do choose to go Gigabit Ethernet, the most common implementation requires high-quality cables with all 8 conductors present with all 8 contacts in the connectors. Cable length is limited to 100 meters in length (an American Football field) which is almost never an issue.

Most devices that are serving as the Ethernet Switches are only 100BaseT capable. That is why I said "choose" to go up to that speed. (Some folks, like me, have to go out and buy Gigabit-capable switches.) Apple Routers (they don't make Just a Switch, but you can configure a Base Station as a Switch) are Gigabit-capable.

My modem, router Ethernet ports are both Gigabit capable and I'm using the best wire I could get Cat 6.

...

The longest cable is 50 feet.

If all 8 conductors are crimped into the connectors with all 8 wires in use, you are all set. Cables that use only two pairs of wires instead of four pairs will not work.

Check by using Network Utility.app to see if you are already using Gigabit Ethernet speeds. If you ARE, then you should also configure Jumbo frames. If not, you can manually configure it to Gigabit speeds, Full duplex with flow Control, but if it cannot, for same reason, connect at that speed it will not fall back, it will fail to connect.

Can I do this by eye balling the connector

yes, exactly.

Cables that use only four are rare, but if you cannot make the jump to Gigabit speed, that may be the reason why.

First, you do not have a thunderbolt port on your computer.

Second, it would actually be slower since you are adding data conversion steps to the process. Data speed will be roughly the same, but latency will be increased.