Thanks for the kind words about the tutorials, Watching them all is quite a feat! It means a lot to know people find them helpful.
Regarding GrowFX, you may indeed see some performance hits simply because GrowFX will need to build the trees before they are sampled by Forest Pack. Depending on the complexity of the plants this can take a little time. If the whole scene was GrowFX objects, these times have the potential to add up. There are workarounds though, collapsing to a mesh is one option as you mention, another would be to try GrowFX's built-in Cache tools to would reduce the build times.
To be honest I haven't tested GrowFX Cache performance vs VRay proxies. Even if it does turn out the proxies are faster. you can still use the same sampling trick for scattering variations demoed in the Q&A using an animated V-Ray proxy. You could build the GrowFX object in a separate max file and update any referenced proxies quite easily by resaving.
The fun of GrowFX for me is being able to model and animate my own plants, something that's not easy any other way. If plant modelling is of interest, and you want to stay in 3ds Max, then I don't think there's currently a better option than GrowFX.
I hope that helps, please let me know if you have any further questions.