I had a single poly motor mount in mine when it cut loose (both times), I'm back to OE rubber now, at least for the moment.
Like I said in my first post.. in my case I'm not ruling out installation issues as well as other mitigating factors.. i.e. the blown headgasket on the dyno. and if fool up there wants to argue about sudden pressure increases and what "should give" and what "should not give", and cannot understand why shucking a hg at high rpms would cause sudden exit of ajacent freeze plugs, then fool above should not be allowed to continue in this discourse... I can promise that the 'instantaneous' pressure at the back of the engine when this occurred was substantially higher than 11 psi (closer to 30 + cylinder pressure), and nothing came out of the radiator cap. Think about compressibility and time frames for events like this:
you blow the gasket into the jacket and instantly that part of the cooling system is over-pressurized some 200psi +-. Water as most know, does *not* compress much at all, and you've just added half a liter of compressed air right there.. it will immediately start pushing and trying to get out/go where ever and at that point the closest pressure release points are the rear two freeze plugs as well as the freeze plug at the back of the cylinder head. All of which exited stage left in a terrific display of coolant and steam. the pressure wave from the back of the engine never made it to the coolant reservoir, never popped the "11 psi safety valve".
I have a spare 2.5 bar pressure sensor, maybe I'll hook it up to the proefi and datalog some pressures at various points in the cooling system, and see what's up.