I've tried similar things with varying degrees of success.
I've tried adding a +level adjustment to all damage from both players and monsters. It made combat move a lot faster, but at the expense of players going through healing surges a lot more quickly.
I've also tried having maximum damage on all hits, with double max damage on critical hits + bonuses. (Essentially this eliminated the damage rolls). This also made combat move a lot faster, but also at the expense of players going through healing surges a lot more quickly.
The second case of max damage on all hits, made combat encounters significantly shorter to around 30 minutes or less for many encounters. But some players didn't like this as much. Some players actually liked rolling for damage.
Try out my suggestion and see how it works for you:
Monsters have 1/2 HP and do 1/2 level +1 bonus damage.
Thus, normally a level 6 Gnoll maurader has 84 HP and does d8 + 8 damage. Under this system, he'd have 42 HP and do d8 + 12 damage.
This system has several advantages:
- no change to players! Your players have to do NOTHING different and have no extra math to work with. They are playing 4e as written.
- Monsters last 1/2 as long, but do more damage with each hit (making up for the rounds they aren't alive).
- Players get excited when they can bloody an opponent with a good hit, and strikers can potentially 1 hit kill something with a Crit (yeah baby!).
- Players feel the hurt when they get double-tapped by a pair of monsters. Using the example Gnoll above... a character that gets hit by 2 in one round is taking 2d8 + 24 damage, plus bonus damage from monster abilities (gnoll pack hunting). That's a serious blow at level 6.