However, general tuning is easier than this, here's what I would suggest:
1. Run at -O to establish baseline performance
2. Run at -fast -xipo=2 -xtarget=generic[64]
If there's no difference (or no significant difference) in performance, then you can stop. [But still profile the application!]
If there is a difference, then I'd evaluate performance gains due to the following flags (some combinations may be missing):
3. -xO5
4. -xO5 -xalias_level=basic (for C) compatible (for C++)
5. -xO5 -xdepend
6. -xO5 -fsimple=2 -fns -xlibmil -lmopt
7. -xO5 -xipo=2
I think that covers the bulk of the things that get enabled at -fast. Hopefully from these runs you'd be able to isolate a set of flags which gives you performance.
You might also want to look into profile feedback for codes which contain lots of branch instructions (or calls).
Obviously profiling the application (eg perhaps with spot http://cooltools.sunsource.net/spot/) will give you insights into what the actual performance issues are, and these insights can guide you to selecting appropriate compiler flags.
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