I posted the 50% hp, 150% number of monsters idea way back. That has worked reasonably well for us, but next session I'm going to try 50% hp, 200% damage instead. Mainly to make the game quicker and more dramatic; fights now seem a bit too long and a bit too safe.
Reading that 25 to 50% extra damage has done it for you makes me a bit nervous tough - maybe 100% extra damage is a bit much. But it ought to work out pretty much the same at 50% more monsters, mathematically.
My reasoning, very schematically, is this:
50% extra monsters at half hit points
Monsters do more damage because there are more of them.
150% damage (numbers) * 50% time (hp) * 150% total ho (numbers again) = 113% total damage over the fight
100% extra damage at half hit points
200% damage * 50% time (hp) = 100% total damage over the fight