I'd advise you look at the optimiser.
We migrated from 12.5 to 15.5 last year. 99% of the code ran ok but we still have a lot of bad query plans - 30 or 40 procs. After the dump and load, try rebuilding all the indexes or at least those on larger table. It will help the new optimiser.
You can also set backward compatabilty to 12 optimiser (we didn't find this worked well in our case).
Instead we found setting "optimization goal" to allrows_oltp worked better.
Other than that we found little difference between 12.5 and 15.5.
For performance, if you're using a SAN, increasing the page size to 8 or 16k helps a lot.
Incidently, why move to V16 ? 15.7 has been out longer and probably a lot less bugs.
Personally, I've stopped using version of ASE until they've been out for a year.
