83% of rated load on 30 AMP 240V dedicated circuit - safe?
Nope

The heater designer was super dumb for designing a heater that is a skitsch over the 5760W limit for a continuous load on a 30A circuit.  Most other heater manufacturers build to just shy of the continuous circuit limits, like the apocryphal 23A that almost every water heater and dryer is built to.  But they may have built to 6000W because it sounds cooler than those (circuit limit compliant) 5500W heaters.  That jibes with their strategy of selling direct to consumers via big-box stores.   

Now you have to pay the price for that.  

On the upside, your working circuit limit for your inevitable 40A circuit will be 7680W, so you can squeeze another 1500W heater in there.  

You could go back to the heater manufacturer and say "are you super sure your device is 6000W and not, ahem, 5600W" like every other heater intended to work on a 30A breaker? 

Probably not, though.  You must use it in accordance with labeling and instructions, because that is how it was tested and approved (listed). So if it says use 8 AWG, there you go.