metal backings are really good. use them! i'd find it hard to see how they'd reduce braking power, but then again, maguras seem to not care about newton and his stupid laws... and about the whole deng "warranty broked if make use CNC backing yes for trials", the same goes for magura as far as i know. just use them, the risk of a snapped piston is quite slim. i've snapped one since i started using the first pair of heatsink prototypes in 2005... i am still on the same pair of backings, so they're quite durable.