I thought the same thing in your previous post. Slicks! You mention the MTB not being efficient I can't figure that one out.
I have a TREK 8000 and ride it on the road at times with knobbies. I also have 28 spokes and surprised that the wheel stay tue after some of the beatings I put on them. But I am only 250 lbs. I did buy a back up set with 32 spokes just ready for the day these puppies go. But I bought my 8000 in 2006 and it hasn't happened yet.
You had $2000 to spend, buy a better back wheel, maybe something handbuilt, 36 spokes. Velocity Deep V does make a mountain bike version. I have Deep V road rims on my tandem supporting 400+ lbs.
Buy the rim and hub then have the shop build it.
I see mtb rims are available in Deep V (30 MM) and if not, the Fusion (25 mm)
http://www.algercyclery.com/veldeepvmoun.html