A lot of effort has gone into testing 15.7, and as a software vendor SAP/Sybase is doing its utmost to make sure there are no negative upgrade issues. But you should always run your own tests if your requirements are so stringent.
There have been many enhancements in ASE 15.7 vs. 15.5, so it will not be exactly "identical". Having said that, any changes should be improvements: better query plans and better performance.
The biggest change is the new threaded ASE kernel which should certainly be helpful for Cluster Edition. But you can always configure ASE to keep using the process-mode kernel that was used in 15.5.
I will leave it to others to comment on their specific version migration experience (haven't done that specific one myself).