Hi mate!
The associated springs fit fine, from memory I think they are a tight fit around the shock bottoms, but they do fit and work well. The included shocks are great - as for some drivers using Associated shocks, like neil cragg, I know they run the rear shocks and different (higher) rear tower to alter the handling.
But to put it into perspective all the box-standard BX cars at the first UK national were in the A final (apart from mine of course! hehe), the ones with modified rear towers / associated shocks etc were in the B final.. So - stick with the standard stuff!
I'm not sure about shock pistons but maybe that is another reason the team drivers might run AE shocks, since the BX comes with unknown pistons and no additional ones for tuning - same for springs. They all know the B4 shocks - so no experimenting is needed.
The BX shocks are close in many ways to the AE ones, so perhaps the AE pistons fit- anyone know ??
Thanks for the kind comments also!
and good luck with your BX if you get it - very nice to build.