You probably haven't considered it, but the main difference between the two quilts is not made up in the stuffing. It helps; however, the difference comes down to how they are made.
As an example, why not just buy the 40* one and pay for a bunch of overstuffing and save yourself more money? Because, the difference is in how deep the baffles are. You have about 2" to 3" of baffle loft on the 40*. You have about 4" to 6" of baffle loft on the 0*. As you'll find out (if you research it), there is a point of diminishing returns on overstuffing.
As far as the temperature ratings of HG stuff. There are actually a little underrated (meaning that you could likely go colder); but understand that this means that you actually have to have it dialed in properly (which usually takes a few tries).
As far as adding a WB protector to heat it up? With only a bottom one, I don't think you'll gain 20*. I bought a 2QZQ UQ protector for $35, which is essentially the same as WBs. I might get 10* out of it, but it's predominately to keep the water off the bottom.
If you want an "all weather" solution, then get the 0* Incubator and vent it, like others have said. I've tried the 1/2, 2/3 and such and they didn't work as well for me as the full length, full loft UQ.
I don't think you'll find too many people that would tell you that you made a bad choice with this. I have a couple of friends that have told me that they wished that they had done this to start...