I would ask PBU about both pistons, the flat top & C1 pressure relieved piston. Are they both 15:1. You can email them or call them on the phone. I have some of there inhouse machined stuff, alum billet rear gear on my X18 and Cag, L7 billet alum torker exhaust manifold, for spring mounted HP pipes.
My C1 is 50cc ( 46cc ) 39mm bore and them piston kits are for 39cc, 37mm bore engines, so I'm screwed there.
From what I can remember, The main advantage with that piston is the "Boost" holes, you can see where they have been cut out of the piston.
I have not used the piston from the picture, but it looks like it could be helpful...
The single piston ring can add some RPM's but will probably cost some power due to a loss of compression ( I'm getting something like 120 - 130 psi, with the flat top 15:1 single ring) but, if it brings the "Squish" down it would probably be a noticeable difference.
ok recieved an email today from pbu telling me the would ship to the uk, but they stopped doing the flat top piston and copper gaskets, so it looks like i will be going with this new piston!, so time will tell!!!
PBU is cool, never got a bad part, they probably have a computer controlled milled w/ like a 10 tool turret, on a hydraulic mount, the part moves and it can choose any of 10 end mills or other tools and they are so fast. We had a machine before that made $1,800 of parts for IBM in 1 hour.
I would go to a bearing shop and match a pin bearing thats good Quality, I don't think your piston comes w/ a pin bearing. I ask them about there bearings before and there the same lousy bearing from China.