So lets state what we've decided here in order to not be redundant. Kid Buu is weaker than Super Buu power level wise, but he's much more persistant and unyielding, making him 'stronger' in a DBZ sense (he doesn't have those long speeches before killing someone).
Ideally, Kid Buu could be the final transformation with no added PL, but given bonuses to Max Ki, Stamina Regeneration, Max HP, HP regeneration, and... Turbo? Not sure if you'd want to create a unique code for just Kid Buu with Turbo.
Now the reason this can't be done is that PL dips a little bit into everything. If you lower the PL gain, no matter how many other balancing agents you throw in there, he'll do less damage and receive more damage all around the board. It'd require way too much coding to make up for it, just to make Kid Buu weaker but stronger than Super Buu. Time is money, friend! ^_^
The only way for this to work out would be if PL had less of a grip on overall game mechanics, making less changes needed to counter-balance the difference.
But as it stands now, PL affects everything, and the rest is complementary to it, from my understanding.