Are main and slow jets same ? If 25 HP carbs happens to have wider internal fuel passages than same 20 HP carbs, you're done, will enevitably need to buy 25 HP carbs along exhaust and consequently kick in 500. Check both advance timings, probably same carbs for both, and + timing + exhaust on the 25 HP model ?
Upping jets on a restricted carb doesn't work and engine will perform awfully. Encountered same issue upgrading a Evinrude 9.9 to 15. $350 after, worked beautifully...