Shield bashing Goliath and large shield spikes

[Marshall]

Or you could suspend disbelief completely and allow oversized light/heavy shield bashes without a corresponding penalty to attacks OR bonus to AC because of the shield's size. So you'd get the higher damage die of bashing with a big, honkin' shield, BUT it's big, honkin-ness doesn't improve your AC nor does it encumber you anymore than it's weight alone does.

Thats what I thought.
This is the RAW answer. There is no penalty to the AC bonus/ACP for using outsized armor and Powerful Build eliminates the penalty for using an oversized weapon.

Looks like it works just fine to me.