Having really good performance with X3b here. Is X3c recommended, or are there enough issues to put it off for a bit?

I fully recommend it. I downloaded it installed it and it is very nice indeed. 

If it ain't broken don't fix it?

But it COULD be broken on your particular setup:
 
http://www.cakewalk.com/support/kb/reader.aspx/2007013331
 
 

Is there any way to install the latest patch separately so that we can compare it with previous ones and can go back if, for example, a new bug is too serious to ignore?

You can always roll back to a previous install, provided Cake haven't changed their update routine
 
You just need to drag the correct SONARPDR.exe file to the correct location (after moving the existing one)
 
But this won't let you do a direct comparison between versions.
 
Just install X3c and go with it unless it introduces problems. Then your first port of call should be here, and not go buggering about with old .exe files

I agree that generally speaking it makes sense to update to the latest version because of bugfixes etc.
But the OP specifically states he's having really good performance with X3b, and we have seen a few people mentioning issues with X3c.

Thanks. I'll install X3c.

No issues with X3c here. CE's and EQ working fine.

Geo524
No issues with X3c here. CE's and EQ working fine.

Do you have 64 bit double precision engine ticked in Preferences?