Kind of an odd question. Losing power as in what exactly? Parasitic losses? Torque management (not really a direct function of the TC anyway)? I'm assuming you mean the torque management system and it's probably a significant amount. Figuring the PCM can cut your throttle by 30% or so if it detects slippage (or to prevent it in the first place).
I would assume that it would be at least as efficient as one from GM and I think they typically lose 3-5% (vs stick shift). I'm sure somebody has more info than me though. Wouldn't a 6 speed manual with a granny low and tall overdrive just be killer???