Can I upgrade to an iMac i7-4.3?

The fastest Core i7 Apple installed in a 27" iMac was a 4.2Ghz "Kaby Lake" (7700K). Using the Mactracker database to find benchmark scores, I find the 4.2Ghz tests at 33% faster than the 3.5 "Haswell."

I've done a bit of processor upgrading in my life and find you need about a 40% increase from the upgrade to be noticeable in everyday use. Unless you are rendering video or compiling apps, you won't notice.

The better question we can ask you is "why do you want to change the processor?"

Is your iMac slow? Easily 95 percent of "slow iMac" posts here have nothing to do with the processor or the RAM, and everything to do with the storage.

Apple offered three levels of iMac storage:

• mechanical hard drive: usually a 7200rpml SATA 6 drive—max possible read/write scores areunder 200MB/sec and common as low as 120-140MB/sec. Thisni the the slow one.
• Apple Fusion (hybrid) drive: if healthy, this can do writes at 500-900MB/sec and reads up 2000MB/sec
• Apple Solid-state Drive: read/writes in the 2000-2800MB/sec range.

A good start is to tell us what storage system you have. Better yet, you can document the "slows" for us with hard data, which will make this go much faster. You post a system config snapshot. Fortunately there is a safe, secure way to do that without our playing a protracted game of "20 Questions" that could go on for days.

We can quickly and within the confines of these forums help you determine what issues are at play if you use EtreCheck Pro, available here:

https://etrecheck.com/index

The free version will do nicely for this purpose, although the app is worthy of our financial support.

We can see hard data about drive performance, software issues, and RAM usage. Etrecheck is the development of a long-serving and trusted contributor here expressly for displaying information in these forums to help us help you. It will not reveal any personal or secure information.

Because the forum software has charcter limits, please see this excellent user tip on effectively posting long text reports like EtreCheck's.

How to use the Add Text Feature When Post… - Apple Community 

You might want to do an online search. Even if it was possible, you can't run anything newer than Catalina which is no longer supported. Plus, the performance increase would be negligible.