Hi,
The way the air ride kit works has seriously put me off it, pulling on the bottom balljoint on every bump is going to cause problems I think.
Having seen the mini world article linked to above, I gave Rayvern Hydraulics a ring this morning.
Their kit replaces the standard trumpet or hi-lo with a ram and you don't need to cut half your rear wheel arches out.
The kit lead time is 2 days, air ride I haven't heard from in 3 weeks.
You retain all Mini suspension other than the trumpets so it will still handle exactly the same as before.
Each corner has it's own solenoid so you won't get any strange cornering behaviour.
£1000 Inc vat gets you sorted other than the control side, compared to air ride where you still need a compressor and air tank.
Cheers