Would the improvised weapon be used for the damage of a shield bash? Or have I just overlooked this written somewhere. Also would the different sizes of shield go varying degrees of damage? Or should they.
consider that the spiked buckler in set 2 lists its damage as 1d6. i think that it would devalue that if you used improvised weapon for shield bash, I'm of the mind that shield bash should be 1d3 to keep it in line with the spiked shield.
I think I'd stick to the Improvised Weapon Shield Bash (which I've even incorporated into my charsheet by now) - Improvised Weapons still have that -1 damage compared to the Spiked Buckler, and considering that an attack with the shield already penalizes the wielder by having him sacrifice the (likely much higher) damage from his mainhand weapon, I don't think it needs to be gimped any further. I also don't see why a shield bash should do less damage than attacking with, say, a wooden stool.
Because even heavy framed oak-wood stools, explode into splinters when you wack someone on the head with them. This is evidenced in any given good-old hollywood western. Shields obviously lack this feature.