Well, you ordered it from aerostich so you can always send it back if these guys have convinced you to go another route. On bikes with the speedo driven from the front wheel the Sigma can answer, but with a computerized speedo I suppose there are better solutions.
If you got the thing and want to make it work, the bracket is the best path.
My old TDM had a very inaccurate speedo, then the needle broke off! I bought a Sigma from 'stich and got it set up (takes some fiddling), then programmed it. Very accurate and I had a KPH display for a tour in BC. Coolness!
The magnet flung off. Had to epoxy one to the wheel. Then a year later,the thing died. I replaced it. A few months later, that one died. Hmm..
Both times it when I was out riding the twisties out of Fossil, strange...
It turns out that I was cooking the pickup as I had attached it to the brake caliper. Most of the time it worked fine, but charging hard and braking to the apex got the caliper hot enough to kill the pickup!
I got a GPS after that, though relocating the pickup with a bracket would have solved the problem I think.