I had a similar issue on my Giant carbon frame and my Garmin sensors.
What I ended up doing is mounting the cadence/speed sensor upside down and backwards. Careful adjustment then allowed the cadence and speed magnets to work properly and allow the speed arm of the sending unit to clear the spokes.
To get the cadence working (which was more important seeing as how speed is measured by the GPS anyway) I had to put the flat rubber spacers that came with the gps under the cadence magnet. The spacer lifts the magnet just enough to make everything work, but not too much that the magnet hits the sensor.
Clearance tolerance is very tight, I am probably only 1 or 2 mm max from anything else.
I had to fiddle around with adjustments for at least an hour.