Great job on the axe and the sheath Fergus, thats a massive amount of work to take on by hand and youve got a beautiful tool to show for it.
My Banko also had a distinct hamon which Ive emphasised by pickling in acid, its not a pretty hamon but nice to see anyway. Still quite tempted to get another one of these myself but dont relish all the grinding again, even with power tools!
I agree with Mojofilter regards the temper, you'd need to go above 200C to ruin it, but I work on the pinciple if its getting too hot to hold dip it in a bucket. The thicker parts of the head dissipate loads of heat anyway and are less critical, its near the edge that you need to be really careful with power tools and within that narrow hardened zone.
P.S. I recently dinged the edge of mine, a clumsy ill considered stroke on a pebble beach in Scotland (how gutted was I), only a gentle touch on the pebbles caused a shower of sparks and the inevitable damage to my oh so carefully honed razor edge, oh well, back to the grind stone