yeah i would wait until your 21 for Shannons, our rates for under 21s can be hooooogeeee!
I did hear, however, with HBF that if you fitted, for example a Power FC, they wouldnt cover it as you cannot change the ecu. Wouldnt SAFC, something that "tricks" the stock ecu, be a similiar thing? Dunno, just just make sure they are happy with that.
Matt
P.S with HBF, are you insured as the main driver of the car too?