I’ve been using a freeware Toronto enhancement pack by Roman Design (he’s built 2 other outstanding Toronto area airports that are currently in the marketplace - best 3rd party airports I’ve seen to date). The guy did an incredible job on it. It literally has zero performance impact that I can notice, and in the Toronto area without it now just looks plain wrong.
I’ve also tried a few others in the Florida area that are also quite outstanding with minimal impact if any. At least none that I would describe as a performance killer. Same for some Montreal scenery I’ve seen. Very good quality and optimized well.
So I know it can be done. I’ve seen it and used them. It’s just that so far, I’ve seen more bad Google rips than I’ve seen good ones.