My (admittedly limited) understanding is that piston ports can actully make more power – albeit in a relatively narrow band – it’s all a matter of what the purpose of the engine is designed to do. A piston port designed for a lot of top end power, with a pipe (remember 2-pipes play a huge role in 2-strokes) designed for that could potentially produce higher peak hp than a reed or rotary?
One problem with a piston port is that it is relatively inefficient in the lower to mid rpm range, but is apparently quite efficient at high rpm’s. When a piston port is designed to be used for karting (and most other) purposes, my guess would be they sacrifice the ultimate power available at the top end for increased bottom end. What good is top end power if you’re a slug out of every corner and never get to use the top end power?
Taking that a step further, I’d also guess part of the reason most PP classes tend to run clutches is to run the engines where they’re designed to work best – basically bypassing the low-mid range where they suck. I’d also guess that reeds and rotaries are generally run as direct drives because of their wider useable power bands.
Stihl is mostly involved in chainsaws, where I would guess that purpose, combined with design and packaging limitations (i.e. where do stick a tuned exhaust?) could possibly allow for a piston port to produce more power than a reed?