My gut feel is that the difference in mount strength is a wash. The lower mounts have less contact area but appear to be integral with the drop-outs, whereas the top mounts have lots of contact area but appear brazed on.

I would not use standoffs. That seems very scary, for the reason you mentioned

Worst comes to worst, you might be sweating for no reason. I doubt have having your luggage 3" higher will make a huge difference in handling. Just pack the bags with the heavy stuff at the bottom, problem solved?