I'm new to owning an NX5 RS so I do not have any info on using different pistions, although I have heard and read plenty of stories regarding problems with Woosner and Vertex pistons. I'm lucky enough to have a couple of sets of HRC domed pistons for my RS and for all the miles I'll be doing they will probably (hopefully!) last me a good while. In fact for what I'll be using my RS for (trackdays mainly) I would think that any of the pistons mentioned above would be okay as long as the clearances are okay and the bike is set up on the 'safe side'?
As for NXA barrels, the basic dimensions of the NXA barrel is the same as the NX5 barrel. I do not know about port heights though.
You do have to machine the bottom of NXA barrels to suit NX5 crankcases.
I would think that machining down barrels to suit pistons is a bit extreme thought. No?