Absolutely oil squirters cost some power. But I doubt it's too much. It sprays with the piston going up, as well as going down. So it's like a windage loss. It's useful for sustained high output on super/turbocharged engines, because the amount of heat the piston must absorb can be over twice the amount of heat a NA engine will ever see. If you allow the piston to get too warm, cracking becomes an issue, and if you spray nitrous, it can actually ignite the aluminum in the piston if you get greedy with the boost.
It's a necessary evil for some applications. Until they come out with useable ceramic pistons or other technology, it's the best band-aid available.
Beryllium pistons anyone?