The seals that come with the can are standard low-friction industrial hydraulic/pneumatic seals. Their rated working pressure isn't an issue, however the 300psi limit specified by Fox may be due to air can strength, eyelet strength or their own seal rating - we simply don't have that information. We built the cans to be perfectly fine at 350psi, however if you run more than 300psi then we can't be responsible if anything does happen to either the seals or the eyelet. We don't officially recommend anyone doing this... but at the same time we're not about to stop you.
Funnily enough, the Corset was first developed on the 66C. It works very well on that bike. Three of our development riders have tested on that bike at different points in time (me included). Notably however, the 66 has a very slack seat tube angle, so the additional sag can be detrimental to climbing particularly steep technical stuff.