I have Dirtbagz too and they work fine without adding too much weight or stress to the subframe. Some little toerag kicked my bike over and left her lying in the street and the metal frame gave good protection, also they're strong enough to use to pull the bike around. The bags seem to be waterproof, well made and strong enough. So be on the safe side I use a bungie on each as a shock absorber. No problems with them at all and they've seen a good amount of use so far in various conditions. IN terms of subframe strength, the cross beam supports where the pillion sits so the only issue would be to load weight further back. We did 6000 miles around Europe two up with heavy equipment with no issues, I'm currently 3500 miles into the middle east, again no issues. Just be careful not to load any weight too far back and I'm sure you'll be fine. The steel subframe of the later X-country is slightly different in design, welded pegs for one. Some racks might not fit. Touratech also make a steel frame and box kit, it's not much more money. I was quoted £800 for the BMW frame, £1100 for the touratech with boxes. THe TT one does not have rear footpegs if memory serves. I couldn't find a single solution that gave me everything I wanted so I just spent £300 on Dirtbagz and the rest on beer.
My only issue was getting them to England where I got stung by customs. Otherwise they were the cheapest option and have worked out great.
