Yes, g3.5 is infinitely expandable, you can edit aircraft to tweak the performance, there is a big community of people who write new aircraft for it, you can even take your own panoramic photographs and create photorealistic flying areas for yourself and others. It is very cool. Some people (like me) think 3.0.448 has better more realistic flying physics, but 3.5 has all the cool stuff like night flying, etc.

I too am not a fan of the s75s included in the DX6 package. However I have a BCPP with the same servos and they are just now beginning to get flakey after 40 or 50 hours of flight time.
I am going to use HS55s to replace them, I have used 55s for many years in airplanes and like them. I would not use them on a 450-class machine, though.