I thought it looked too good. Though we have a large group of.... well characters making attack rolls (some melee, some ranged, some spells). Since I'm primarily a fighter dabbling in rogue here and there, I'll probably have 4 levels of rogue around 14th level.
In an iconic group of say fighter, cleric, rogue, wizard... It would be quite balanced and wouldn't raise an eyebrow. The fighter can power attack, the cleric will hit (if attacking), the wizard would hit (if attacking with ranged attack spells).
If you are into house-ruling, have a max -5 AC, or -1 AC per 1d6 of sneak attack sacraficed, or have the -5 AC apply to one ally for each D6 sacraficed.