Stripped threads!? Did this just happen or are you thinking that this is the cause of the power loss? Loss of compression would certainly put the hurt on the power output.

Going forward...be careful tightening the plug! A good "snug" with the plug wrench (finger pressure...not bicep) is all it takes. A small screwdriver style "nut driver" is good for this...limits the amount of force you can apply.

Also...when installing a plug, it's easy to get cross threaded. I've found that a 1 or 2" piece of fuel tubing slipped over the electrode on the plug makes a handy installation tool. The plug is gripped by the fuel tubing making it easier to install it into the often recessed plug hole. In this manner, it is not possible to damage the threads. Either the plug installs easily or...the tubing simply slips. When it's in...snug it with the wrench/driver and that's it.

Good Luck!