Since it's supported by SAP, is it reasonable to think that we could open OSS incidents for code which runs slower on HANA?
Of course not. The transactions still work, so there is no malfunction present to be fixed. And this is what support incidents are about.
And while the code might not comply to the performance wizard's recommendation, this doesn't make a bug to be fixed either.
As written before: changing the SAP code is a modification and for a platform change that goes beyond a runnable solution (incl. considerations on how to best leverage SAP HANA as a platform, changing data structures, dropping indexes etc.) we should have a chat and see what the best way for this specific customer would be.
With SAP Custom Development we're in the inverse situation of the standard support - here every project is unique and common answers may not be as easy to apply.
So, in practice: just get in touch with us on that topic and we'll have a discussion on how to move forward here.
Cheers,
Lars