I am aware of some the obvious gains of the x64 architecture (higher addressable RAM addresses, etc)... but:
As to performance issues, it depends on your program actually. If your program is pointer-intensive, porting to 64-bit may cause performance downgrading, since for CPU cache with the same size, each 64-bit pointer occupy more space on cache, and virtual-to-physical mappings also occupies more TLB space. Otherwise, if your program is not pointer-intensive, its performance will benefit from x64.
Of course performance is not the only reason for porting, other issues like porting effort, time scheduling should also be considered.
I would recommend porting it to 64 bit just so you are running "native" (Also, I use OpenBSD. In their AMD64 port, they do not provide any 32 bit emulation support, everything must be 64 bit)
Also, stdint.h is your best friend! By porting your application, you should learn how to code portably. Which will make your code work right when we have 128 bit processors too (in a few decades hopefully)
I've ported 2 or 3 things to 64 bit and now develop for both (which is very easy if you use stdint.h) and on my first project porting to 64 bit caused like 2 or 3 bugs to show up, but that was it. Most of it was a simple recompile and now I don't worry about the differences between 32 and 64 bit when making new code because I just automatically code portably. (using intptr_t and size_t and such)
64 bit will run a lot faster, when 64 bits compilers become mature, but when it will occur I dont know
