This reply may be too late if you don’t have the R 80 any more, but by fitting the brace to my leading legs on my R model to stiffen it all, the problem with the front brakes on tight right hand corners is no longer a problem.
I’m not sure if it comes from the forks/L.legs flexing or from the wheel bearings allowing the discs to mis-align a bit. esp. if they are tapered rollers.
I’ve used 20mm/2mm square steel tube, well re-enforced with gussets in both corners. I could have used 25mm/3mm tube which would have been even stronger but heavier. If I was fitting such a U-shaped brace to a K model BMW, I’d certainly use the 25mm square tube.