wheel spin shouldn't be a factor. You are getting too worked up over the dyno numbers honestly. Each dyno will be different. If you wanted big numbers than you should have gone a different route. As long as your car fells good when you drive it than don't get so worked up in this. Your dyno will make a huge difference since Mustang dynos, dynodynamics dynos and dynojets all do things differently and give quite different results.
335i coupe Montego Blue Dinan stage 3 tune, Advan RS wheels 350whp/365wtq
1994 Brooklands Green Acura NSX Comptech Supercharged 390whp/300wtq