Ranger level 1 using a longbow with Twin Strike and Hunters Quarry does 2d10+1d6 damage. Every round. No stealth check.
Warlock level 1 with optimized Cha and Warlock's Curse does 1d10 +5 + 1d6 damage. Every round. No stealth check.
The 'perfect balance' to be achieved would be to come to some sort of balance where the rogue gets his +2d6 damage from combat advantage every other round on average.
And this is easily arrived at by saying when you attack from hiding, you lose your hiding and can't hide again for one round. Like you propose, attacking makes people 'notice' you, but that 'noticed' state only lasts until you've had some time to move around and the target has had to focus back on the combat in general; which is to say, the end of your next turn.
This seems fair to me
Edit -> This is assuming that the Rogue's stealth is good enough to alomst always trounce the target's perception - which, at least all through KotS, it always is... everything in there has a whopping +1 or +3 perception, with the great exceptions being elites with +5 or something, while our rogue is running around with +12-14. Easy.