Yep. If it's just 32->64 there may even be a slow down as you need to pull bigger things through the various busses. However, Armv8 also brought a huge improvem

Yep. If it's just 32->64 there may even be a slow down as you need to pull bigger things through the various busses. However, Armv8 also brought a huge improvement to the instruction set, so recompiling with the proper optimizations will bring performance increases in a lot of programs.