I cannot say if it applies to your case, but 30W@54.6V mean 550 mA.
From what I understand 802.3at means “up to 600 mA” (while 802.3af means up to 350 mA), and Mikrotik provides max 450 mA as there is anyway the limit in the standard to 25.5 W per device and 25.5/0.45=56,67 V, which is in practice the max voltage of 57V, so it is 802.3at compliant see:
http://forum.mikrotik.com/t/only-450ma-poe-output-for-802-3at/155237/1
I wouldn’t be surprised that your AP expects to get 600 mA to consider the power source “fully” 802.3at (while in reality one would need 802.3bt to exceed 25.5 W)-
In any case if the AP really-really is 30W, it needs 550 mA@54.6V and the hex Poe per specs won’t deliver that amount of current, and you have to consider anyway some “slack” or loss of voltage.