I've been doing some math.
Here's the score needed to hit a monster of the same lvl
Code:
PC SCORE TO HIT (same lvl)
1: 9
2: 9
3: 9
4: 8
5: 9
6: 9
7: 10
8: 9
9: 10
10: 10
11: 10
12: 10
13: 10
14: 10
15: 11
16: 10
17: 11
18: 10
19: 11
20: 11
21: 12
22: 11
23: 11
The maths seem not bad if you presume that most of the time, players will get +2 either from a synergy bonus (from a power) or from Combat Advantage. Without any exterior bonus, it average between 55% to 45% chance to hit, and 65% to 55% with a +2 from synergy/CA bonus (wich can get higher if you get both and/or gives the monster a -2 to hit).
If your party takes on higher lvl monsters (wich, in my experience, happens all the time; someone already mentioned the average monster lvl +3, wich seems correct), the chance to hit drops by 3 (15%), going down 40% to 30% and 55% to 45% with +2 synergy bonus, 65% to 55% with a +4 (rare), 85% to 75% with +6 (extremly rare).
30-40% chance to hit seems very low to me since those maths are calculated from optimized characters (but not including feats).
I see then two solutions.
1: Either adopt one of the solutions in one of the [enter a two digit number] posts already in the forums.
2: DM use only monsters of the party's lvl.
The #2 have the advantage of not having to change the system at all, and fix the attack and defense problem at the same time.
One question remains though : will same lvl monsters be challenging enough (HP and damage dealing wise) ?
I'll try to use same lvl of the party monsters, but using more monsters (using the xp pool of 1 lvl higher and as if the party was made of 6 players instead of 5).