Let's start from something close to 4e math, like:
PC: 25 + 5/level hp, 6 + 2/level damage
Monster: 24 + 8/level hp, 8 + 1/level damage
PCs hit two times out of three, monsters once in three. PC vs. PC and monster vs. monster would be 50% I guess.
Are there problems with this?
I think so.
Level 1, PC takes 4 shots to kill foe or 6 rounds. Monster takes 3.3 shots to kill PC or 10 rounds (course, 3.3 shots in reality often means 4 successful shots or 12 average rounds because 3 shots doesn't normally cut it and requires 1 more hit to really work).
Level 5, PC takes 4 shots to kill foe or 6 rounds. Monster takes 4 shots to kill PC or 12 rounds.
Level 30, PC takes 4 shots to kill foe or 6 rounds. Monster takes 4.6 shots to kill PC or 14 rounds.
As can be seen, as the PCs get higher level, the monsters take longer to kill them. Combined with PC synergies, monsters here are not much of a threat at level one and their threat potential drops as the PCs climb in level. Btw, this same thing happens in 4E as well. 4E is off the balance chart as one gets into higher levels. The only thing keeping PCs and monsters even semi-balance in 4E is the fact that PCs do NOT gain 2 hit points per level. Many PCs do not even gain 1 hit point per level in damage, hence, the monsters don't die quite so quickly. PCs instead do multi-target and striker damage. If a high level party in 4E does not have multi-target or striker capability, they tend to die eventually.
Level 1 PC vs. Level 5 monster, PC takes 8 shots to kill foe or 12 rounds. Monster takes 2.3 shots to kill PC or 7 rounds (course, 2.3 shots in reality often means 3 successful shots or 10 average rounds).
Level 5 PC vs. Level 1 monster, PC takes 2 shots to kill foe or 3 rounds. Monster takes 5.5 shots to kill PC or 16 rounds.
Here we see that the low level monster has no chance to threaten (5 to 1 ratio in rounds) the PC (because of your adjustment to the OP's model of giving PCs twice the chance to hit monsters than monsters have to hit PCs), but the low level PC has at least some chance to kill the monster (closer to a 5 to 3 ratio or even 3 to 2 ratio).
Personally, I don't really consider this that balanced. The PCs wipe through same level foes with ease (without taking into account Encounter powers, AoEs, or Striker abilities, basically just using At Will powers) and higher level foes with some resources.
The monsters have virtually no chance at same level and even at higher level, don't really represent a major threat when one considers other PC abilities (like Striker damage, etc.). Granted, monsters tend to have one or even two encounter power that is a bit stronger, but that doesn't compare to extra PC resources.
And of course, there are the damage issues that have already been mentioned (i.e. How do we do 84 points of damage with just 5 or 6 dice? and Is an 18 Strength worth it if Strength only adds 4 points of damage to this mega-total?, etc.).