Don't forget the Roadster & Plus 4 run bigger tyres too, so proportionally the 22psi is fine with the heavier front ends on these cars.
If you're happy running 18psi, then stick with it Andy. Steering response is theoretically slower and the tyre slip angle not as sharp but unless you're into quick motoring you should be fine.
If your tyre wears a bit, then you'll know earlier than most I would guess..!
I checked mine at the weekend and found my N/S/F down to 15psi & it certainly showed through the steering. I thought it was down to needing a grease on the pin, but no. D'oh!
I'd be wary of going any lower or having a big differential between front & rear I think.
