d8 does seem a bit high. But then you consider that you need to get a feat to get proficiency in it.

I'd say d6 is a good middle-ground. Then it's a d4 with the damage die increased only once.

Basically, the feat gives a +2 Proficiency with improvised melee attacks (specific only to fists), which is pretty incredible by itself, considering you are always armed with fists. (Do you get attacks of opportunity with fists?) I'd say a +2 Proficiency bonus when using your fists as improvised weapons would be a feat in and of itself. You could reduce the prereqs for this though.

A second feat could replicate the Rogue's shuriken ability and increase the damage die by one. (I think that's to d6.)