I've just been offered a set of pannier, and wondered if they will fit the standard BMW GS pannier rails, or do I need to spend the £150 on top for rails?.
You could use the OE frames, but with modification.
The TT pannier mountings are thicker than the tube used for OE pannier frames so you would have to either cut the plastic spacers down or fix something to the frames to make up the gap.
If the panniers are second-hand then you will obviously have to reposition the four mountings.
That said the OE frames are nothing like as strong as the TT ones, it would be best to get the TT frames as well.