Hi Tobias,
you can create a DB-trace with transaction ST05 to see that the "up-to-rows" clause is pushed down to the DB (you will find the addition 'limit' in the actual SQL statement). In the trace there is also an option "Display execution plan" (or "explain SQL") and this plan would provide some info how data processing is done. This may vary for different where-clauses.
Regards, Christian