I found this mod on the Nexus which fixes the bug on Deflect arrows - namely that having deflect arrows gives you 85% damage reduction to all physical damage rather than 100% DR to arrows that hit the shield. The only drawback to this particular mod is that it no longer checks against whether or not the arrow actually hit the shield - it just blocks 100% of arrow damage when blocking. It could be slightly OP, but it's not mroe OP than the current situation, which just breaks blocking entirely. You could also probably load up the CK and adjust the 100% damage vaue to 85% or 90% if you were feeling ambitious.
http://skyrim.nexusmods.com/downloads/file.php?id=7740
edit: just saw someone already posted this
