Well Kyle has apparently just said he's hearing from AIB's that 1400-1600Mhz is very possible on custom cards though it's a bit of a silicon lottery in what it would do.
91C doesn't suggest running hot, or not, it depends entirely on the fan speed/noise being used. 1.5% clock lower was running at 83c and from every other leak it would suggest that level of heat at a low fan speed, none of which points to running hot, just quiet while on a tiny cooler.
A chip itself running hot means a giant heatsink and high fan speed are required to keep it stable let alone cool. A tiny heatsink and low fan speed and running at 83C doesn't suggest hot, it suggests it's running warm on a cheap heatsink, nothing else.