In my experience up to about 900 hp has been good.The aluminum drum won't come apart. Parts I think are a must at higher power level? A good front drum, steel front planet, good quality valve body, 518 deep pan with a long enough extention to get the filter at the bottom, reinforced band strut, 3.8 or 4.2 lever, solid rod front servo or shimmed solid late servo, lube hole for the sprag. Just took mine down for service as there was a few pieces of low roller clutch spring in the pan at about 250 runs. It does not use no low band apply. It shakes the tires once in awhile, so regular maintenance. New inner race, spring set, and a filter. Ready to go back in service. The band has been in the trans for over 700 passes, the clutches for over 250. In fact the old front clutch frictions and steels were moved to the rear clutch last time around. No metal on the filter either. If you need less 1st gear ratio or a aftermarket case at a higher power level? Then it's time for a 400.
Doug