Hell my standard weight XHS repeaters with the Stiller TAC408 receiver and the XLR chassis stock will go well over 20 lbs once scoped with a bipod.
You say you want the rifle as heavy as possible but then you say you want a +20 lb rifle, that my friend is not an overly heavy rifle!!! There are a lot of conventional varmint rifles that push 20 lbs.
Need to decide where you want your rifle finished weight to be and then we could recommend a specific place to go for stock. To me, a heavy rifle is over 30 lbs. For that you would want a weighted McMillan or get Joel to set you up with one of his weighted Laminated wood stocks. The balance will be much better also as you will want to get some weight behind the receiver with a really heavy barrel so that the rifle rides even in the bags and so that recoil will be straightline and consistent.
I really do like the XLR stocks, used a lot of them but not sure they would be the best for a really heavy barrel choice. I would think they would feel a bit muzzle heavy at best.
I use a custom contoured barrel that is 1.350" at the shank for roughly 5" and then contours down to a muzzle diameter of roughly 0.900" at 33" length. With heavy 50 BMG style flutes from Lilja. With the much lighter Stiller TAC408 and a Bruce Baer 7 round DM system, my repeating XHS rifles with the XLR stock come in at around 22-23 lbs with NF NXS scope and 9-13 harris bipod installed.
This is about the heaviest I would recommend for use with the XLR stock. If your looking for heavier then this, you would be better served with a different stock design.
The BAT 2" is a fine receiver but you really have to pay attention to keeping the bolt lugs clean and well lubed. I prefer the Model MCT for my rifles in the 1.55"x8.5" single shot version.
Have made a few heavy rifles in the 60 to 75 lb class range with this receiver and they have worked well other then they are a crew fired weapon system to get into position!!!
