2011 Tundra reduced max tow capacity, options?

So my wife and I have started looking at TTs to determine what we like...we're not buying until next spring, but it's gonna take us that long to figure out the options we like, floorplans, see what's around, etc.

When we bought our 2011 4x4 Tundra CrewMax in August of that year, the spec sheet said we were good for up to 10K. Toyota has since reduced that number to 9K...to me, that's a huge difference? What really stinks is that one of the TTs we like is the Cougar High County 321RES, but it weighs in at 7865 shipped weight (I can't find a UVW or GVWR on the Keystone site).

Being completely new to this, and having never towed anything besides a couple small utility trailers (sub-1K) locally, I'm hoping for some advice:
1. Based on my math, with a 20% safety margin, I'm looking at 7800lbs max tow weight; is that correct?
2. That 7800lbs should be the GVWR...so UVW plus any stuff we put in the TT, correct?
3. Will a WDH make any difference in the max capacity (a coworker told me that it would...but I'm cautious)?

I've already assumed that we'll get a WDH with whatever TT we get...seems like a no brainer based on my reading here (been lurking for quite a while before posting).

Thanks in advance for your help; I've read dozens of threads and looked at what other Tundra owners are pulling, so I have a good idea, but thought I'd ask some specifics if anyone has a minute to assist.

Thanks!!