I had a 3 day test drive in a Mini Countryman PHEV a few weeks ago now, and I have to say I was impressed by the combined power output of over 220 hp (noticeably more than the 192hp in my 20i petrol/gas X1!) when you put your foot down and both the 3-cyl turbo petrol engine and the electric motor kick in. The Mini Countryman has that same setup you mention; the petrol engine drives the front axle and the electric motor drives the rear wheels. So it's marketed as a 4-wheel drive car, as well as a plugin hybrid. The motor and batteries are all under the boot/trunk floor and the rear seats. The rear seats are a little higher as a result but there was still good headroom in the back. The switch between battery and petrol/gas engine was seamless, and at slightly higher speeds when road noise picked up you couldn't tell which power source was in play.
If this same setup makes it to the X1 PHEV, potentially with a higher capacity battery that is already scheduled for the Mini Countryman in the next couple of months (9.7kWh), then I'd be interested, as long as the price differential doesn't negate the potential fuel savings. I did a write-up of my test drive
here if you're interested.