Just an update, in case it had any interest.
Last night, in my usual campaign (players are level 17), I changed all monsters (so easy with Monster Builder) by removing 33% of their hit points and adding +50% damage (for example, 3d8+7 became 3d8+17).
I was very happy with the result. The fights were fast and furious. The first 3 fights were fairly simple n to n+2 fights. All three lasted between 15 and 24 minutes, including initiative setup etc. The last fight lasted 1h15 but that was *final battle* against a trio of vampire - so lots of regeneration and dominating effects. It was supposed to be long, but I didn't expect (after the duration of the 3 first fights) that it would last that long. Should probably have clocked in around 45 minutes, but I made the list of powers of the vampires way too long and I think I took too much time per round.
Either way, I am still pumped about last night. 15-25 minutes is perfect (for me) for fast and brutal fights, but with a certain amount of tactical options.