Maudib
I don't think many people have much experience with the 45GX so far, being practically a brand new engine on the market. As I mentioned before manufacturers specifications are mostly inflated. So if you want to compare any engine against another one you don't have much "real" data to go with. The most one can do, especially with a new engine is to "guesstimate".
My point in using a 40cc class engine in the GP Yak is a matter of "installation ease". I have a ZDZ 40RV I could have used in this plane, but I am a little adventurous and will like to try something new in this engine class. I am sure the ZDZ will fly the Yak as well, or maybe better than the Evolution, and you can install it fairly easy on the GP Yak.
Probably the easiest engine to install in this model is the recommended Fuji EI-43. BUT what the heck........different strokes for different folks. There is people that will swear by their DA's and I think is more like the Harley against Rice Burner controversy, you know.......is made in America, so it has to be better. And by the way I am not on any one engine distributor payroll. As far as I concerned you can install a Lycoming IO-540 if that's what you like. (It may ruin your wingloading, but the thrust to weight ratio would be out of this world)
By the way I plugged the "real world" data for the DA (23-8 MenzS @ 7100 RPM) versus the same prop at (a theoretical) 6200 RPM on the Evolution. And while the DA shows some 10 Lb more thrust, the thrust to weight ratio difference is only .7 Lb more for the GP Yak wingloading. So...in the final analysis, I am not going to loose any sleep over this controversy.