As your testing please note the following:
KD and dueling, has very little to do with the shielders vast responsibilities on the battlefield, and thus, you need to test against its other situationals:
Can he/she still be affective leading the charge into a group of maulers etc at the top of a ladder?
Can he still provide archer support and anti-cav with a weakened shield that he isnt allowed to hold up anymore without penalty? etc...
I dunno, the only way you can accomplish what you were saying is a nerf, not a buff for not holding your shield up. Logically you'll just have the same people gratuitously dropping their shields a little more often, and them lasting even longer.
Thats why I said all this, because you said your intent was to break up the turtling, this wont do it- you will just buff the turtler slightly, (because they'll drop their shield every now and then, thus making their shield last even longer) and the savy shielder even more.
So i see a nerf coming, not a buff, and I really dont think thats going to help anything. Shielders have it tough in almost every other aspect but shielding, and they arent exactly owning the boards the way it is now, so...