Simply 'upgrading' to a 64-bit version of the software should have little/no performance implications (e.g. you can run a 32 bit ASE on a 64 bit platform and compare performance).
The performance implications are in the ability to use more memory and generally the better CPUs.
ASE 12.5 has a lot of performance and other configuration changes which make it pretty easy to extract lots of performance from it -- or burn it to the ground if you're not careful. Read the release and migration docs extensively.
One change which is huge is that you have to specify exactly the memory useage of the default cache (if I am remembering correctly). Otherwise it is tiny and people complain of horrible performance until they notice this.