Caliper piston sizes

I'm rebuilding a set of dual calipers off a kz1000. They are the 38x40mm pistons. I bought these super cool phenolic 38mm pistons that are slightly shorter at 35mm. I installed them and everything seems like it will work, the piston just has to stick out a little farther. The dust seal fits fine and with the piston out to the point of contact there is a solid 20mm still in the piston bore. With pad wear this might go down to 10mm at the far end. I'd really like to run these as they will never rust, don't transmit heat and are a quarter of the weight. Thoughts?