A i recon a lot of balancing problems on the shield side are on the effect of the overall usability of shields, esp tower shields.
How about the following: tower shields must not be equipped unless you have the feat towershield profiency.
They are designed to cover the whole body, and prob should also take away some spell dmg (hide behind from a fb), but without the profiency its just a pice of junk. The actual penalty for untrained usage is just too ineffective.
I recon this would prob cause some griefing, but it would make it at least possible to add some defensive properties (spell dmg absobtion ae).