Which Katara will help me deal more damage?

Oh wow, good lord. I thought it was a flat reduction, which would make it so awesome. Seeing it's multiplicative it's not as great. A bit surprised that mobs at my 128 range still only have about 10-15% PDR. As for why I'm still using 8x weapons, I made the Dual Blade to be DEXless and only now have I given her some gear upgrades so she can eventually use stronger stuff. To be fair, a DB with 8x gear is still kicking major ass for me.

From what I got from a quick glimpse at the datbase, the only monsters/bosses with PDR as high as the 50%+ range are mobs/bosses involved with Chaos Zakum, Pink Bean, Cygnus, and Azwan. I assume then that THE biggest damage boost I can possibly get is +30% boss damage.

I thought exactly the same thing as you before until someone corrected me which was when I went "What, so you mean Marksmanship does not in fact make me deal 25% more damage!?" And yes, 30% boss basically means something like this:

Take a 50% PDR mob. Let's assume you deal a fixed damage of 20 k for this and let's say you have either 30% PDR or 30% boss.

30% PDR x 50 = 18% so you deal 10k (1/2*20) + 1.8k damage= 11.8k.
30%*10k= 13k.

There are however circumstances where PDR gives you more bang for the buck, such as at Cygnus (70% PDR, I think?).