Kim, are there any signs that the pistons picked up the sleeve? Such as sieze marks.
I have had good results with GPM pistons in 2 Rocket engines and a Norton Atlas as have some of my mates.
There are some here who have not.
In my low compression engines, 7.25:, I have used pistons which came in a plain unbranded box, probably Taiwanese, but looked good and so far are performing well.
As to JP, I recently did some work on an A65 which has new JP pistons fitted. They looked very good compared to the old production heavy things of the past and I suppose that I am prejudiced against the new ones.
The rings don't seem to be up to the job as the engine seemed too smokey to me, maybe time and a few miles will improve it
Trev.