Sorry for the necro, but a heavy shield does d4 on a bash attack; a spiked heavy shield does d6, and does not stack with bashing. A warshield is a light weapon, and does a d6 on a bash attack. Bashing, if legal, would make it deal 2d6 as a light weapon.
EDIT: I double checked, because I looked this up a year and a half ago, and I believe it does not stack because Spiked Heavy Shield has an armor/weapon entry separate from a Heavy Shield, thus it cannot be applied to a Spiked Heavy Shield. If this ruling stands, then the best you could do is an Impact Heavy Shield (minimum +3 enhancement bonus to accommodate the +2 price of Impact). Kind of expensive for a light weapon that does 1d8, but it counts for any shield bashing feat I can think of.
