I think there's a bunch of things to consider. Firstly, the engines need to be powerful enough so that newer players can build functioning aircraft. Secondly, the stats don't seem unreasonable to me. I've heard that the rocket parts are underpowered compared to their real life counterparts to compensate for the fact that the Kerbol system is really small, and I think the jet engines aren't scaled down as much. Thirdly, and most importantly, the reason players can construct Mach 7 upper atmosphere super-planes is because KSP doesn't make engines overheat despite the fact that the intake air would be travelling at supersonic speeds. This allows them to travel at the speeds they do. Personally, I think they should implement jet overheating that would need to be addressed with a new Pre-Cooler part. (Maybe find a use for the Radial Engine Body?)