Upgrading to a QuantumSuit from an Advanced Jetpack is sacrificing speed for the protection that it brings. The GraviChestPlate has both protection and speed, hence it is an upgrade from the QuantumSuit.
That might be an argument, but in vanilla ic2 it only requires a normal jetpack. Also i do not understand why the lappack functionality is lost as well.
I do not like the GraviSuit that much. In my eyes it only creates problems. It is so fast that it creates lots of chunk loading laggs when used. In my eyes the speed of the adv jetpack is just right.
I havent tested it. But is the nanosuit adv jetpack also slower? It might be a case of not only having two helmets, but also two armors: an nanosuit adv jetpack for flight and an gravitysuit for protection and lappack functionality.
But I do not expect that you will change you find in this regard. I guess I have to write my own ic2 addon. competition ftw.