Goffy,
I think that the bloke who does my balancing may be a bit too far away for you.
I have no problems whatsoever with GPM pistons and are running them in my Atlas at the moment.
They are in my RGS and were fine, when it was on the road.
I have used Hepolites in the past and I don't think they were any better. It's a case of using what is available at a reasonable cost.
I have no-name pistons in my Flash, 7.25:1, and they seem as good as any.
Trev.