I'm using the Spider EVO. It does restrict the steering lock, which is not a problem when riding, but doesn't help manoeuvring in a crowded pit garage at all. Yes, the horn has to go, or be repositioned somewhere.
It's also much harder to fit than the R&G - you have to drop the bottom yoke. The R&G ones don't work that well with the FGRT forks because of their scalloped tubes.