I suppose a seasoned developer can at least do hex addition and subtraction in the head:
It is obviously for example to see that DEAD+F00D=1CEBA.
As for multiplication, there is always Karatsuba's method.
In all seriousness though if doing a lot of debugging, the mental palette can sure speed things along if you can handle some simple hex computations and with enough experience most people gain some familiarity there.