I drove Helen's nephews tuned Mini One several times and it was quick Janspeed exhaust, K&N and ECU remap at Falkland performance centre. Rolling road printout showed it had 130bhp. We, erm, "tested" :I it against my last Cooper a few times and it was identical acceleration (more bhp but higher gearing in the One) but I could pull away a little from him in the corners due to my sports suspension (both cars on 15" alloys).
Go for it - it really transformed his Mini One and made it even more fun to drive