Performance ratings
 

When manufacturers of aftermarket parts claim you’ll receive an xyz increase in performance when you use their abc parts, do they usually dyno a stock car before and after the part is installed, to give those numbers or are they usually just guestimates?

It depends.

I think those HP gain claims are true only to cars that already have other performance parts on them that are being held back by a stock worn out part that you are replacing. Best case scenario claims done in optimal conditions instead of real world conditions. Watch those shows on tv like Horsepower when they put stuff like nitrous kits on a car then dyno it before and after and even though they are using a specific HP nozzle, the dyno results are always less than advertised.