I think that the 3D heli physics in RC8 are pretty good, but of course not perfect, lol!
I mainly fly 3D with the RC8 Goblin 700 and when I pitch pump the collective the headspeed RPM changes under load. For simple flips/rolls there is not any RPM change. A real high performance Electric with a good speed controller/governor with a proper amount of headroom and collective travel, in good hands, does not bog down either (or not too much anyway, lol!).
I also use Realflight 8 and maybe it's heli physics are a bit better but not significantly better imho. I have not tried Reflex XTR2 so I cannot comment if it is any better or not, but I doubt it's perfect. However, if you think it is then that's what you should be using I guess.
I think that the main purpose of using any RC Sim for helis (and other aircraft) is to help teach muscle memory. I believe that both RC8 and RF8 are suitable to help achieve this. Of course you will always need to eventually work this out at an actual flying field with a properly setup heli.