There is a reason why Improved Shield Bash is a great substitute. Sure, it only gives you 1d6 instead of 1d8 or higher as base damage with a heavy spiked shield, but at higher levels, that is the last of your worries. With that single feat, you get the benefit of two feats AND an extra equipment slot (shield and spikes are considered separate items). Last, but not least, no one said that you have to wear the shield on your off-hand to gain the shield AC. Wear it on your primary hand and wield your shortsword in the off-hand. No extra penalties.