The few reported sub frame breakages tend to be on the top and in line or there abouts with the first hole, as that's where the bottom part of the frame ends, and where the greatest stress is when carrying a load that bouncing up and down on the back.
If I was strengthening mine, I'd look at adding a small 4mm or so bar a few inches long over the top subframe, thus doubling the wall thickness of the subframe tubing in that area.
The alternative would be to weld a triangle type structure underneath the subframe.
The TT subframe is £423. The technical data for it has not been released so you can only guess as to how much it might be able to carry, but should be in line with the 09 OEM one. You probably don't want to do, but if you ever did want to carry a pillion, you'd need to drill your own holes for the pegs.
BMW have stock of the 09 subframe and you can get one delivered to the UK for around £550. The 09 bikes can carry an additional 30kg according to the data sheet.