Re: Developer warnings option on User Options
It's sat there since v1.5, no one else I know has used this option. The errors and warning that you get as standard are fine.
Errors are compilation errors or faults in objects (i.e. field on a report with incorrect data binding)
Warnings is where the compiler can see a possible runtime undesirable result, from experience this is typically numeric precision.
The performance side is the SQL tab, where AX will popup warning to the user (the user with the option enabled) if a criterion is met (i.e. query exceeding x milliseconds).
I would also do some reading on SQL performance tuning (data management views and the index advisor); although you need to take into account that AX has enforced index columns and you can't alter indexes within SQL server.
AX is, except for DataAreaid, fairly typical so normal rules apply when queries (i.e. never select * unless you need to update, appropriately add indexes to minimise bookmark lookups, etc).
Hope this helps.