At the current formula, [(hp of last MP mobs) * (1.4)], MP15 mobs would have approximately 5.38 times as much hp as current MP10 mobs and MP20 mobs would have approximately 28.93 times as much hp as MP10 mobs. I have not calculated how much damage these mobs would inflict but I am certain it would make higher MP game VERY challenging.
If this was implemented, however, then there would have to be changes to the Ubers. On MPs 11-20 they might possibly drop their regular organs once (never more then once) as well as a small chance to drop something else that can be crafted into something awesome. I also think the ubers should always have a very small chance to drop recipes that can ONLY be gotten from killing them (maybe 1% chance per MP for a recipe to drop).