"I guess my question is, will this hurt anything? Am I losing performance?"
If the windowed piston were used in the factory P/C, it was done for a reason. It provides a path for airflow. In the intake charge we also have lube oil. Lube oil is used to protect the wrist pin bearing and wrist pin, and the air movement removes heat as well.
You loose the airflow and the heat dissapation with the new piston. Does it effect engine power, not sure? You'd have to have done some timed cutting before and after the piston swap with no other changes.
I'm not into porting these engines, or modding them other than minor muffler mods and improving squish if I'm removing/replacing P/C's.
From what I've noticed, the open port designs tend to produce broad/smooth power, and still like to rev, but pull down more in the cut.
The closed port models, enjoy high rpm's, and seem to have stronger power in the upper rpm ranges.
The best comparison I have is between my 51 (open transfer) and 55 (closed). They both love to rev, and the 51 is smooth clear across the speed/load range. The 55 is quite a bit stronger in the cut, and doesn't pull down as easily as the 51. It enjoys and stays at higher rpm's, but seems to have a narrower operating rpm range in direct comparison to the 51.
All of my "Pro" Husqvarna's are closed port, and all of them have excellent power to weight ratios, and strong top end power when in use. Both my 262XP and 268XP easily rev to 14,000rpm's no load and still "2 stroke", but I don't set them that high for firewood cutting.
I had a Husqvarna 61 for a while (1983 model open transfer), and it was a TURD in comparison to my 262XP. I couldn't stand running that saw, sort of like driving Corvettes all the time, then getting into a Geo Metro, so I installed a 268XP top end on it. Completely different saw afterward, and I made the mistake of letting a good friend use it, and I got back cash instead of my saw!....LOL.....Cliff