I seem to recall a thread here in which a guy in the UK had done a whole series of mods to his Roadster after doing very detailed testing of drag. This included changing the mirror mounts. He claimed he had a range of like 300 miles at normal speeds (eg, not doing 30 mph). He also commented that when he took it to the local SC, he removed lots of the mods but they still took notes on the things he didn't take off.