That must be a UK thing b/c I've never heard of that in the states... anyone care to comment?
The only thing I've heard of is tire dealers (ie Costco) not willing to install anything other that the factory sizes, but that's for liability reasons.
I have the sport package on mine which means I also have the sport suspension. These cars have the staggered tire setup from the factory with 18" wheels.
I swapped out the rims and tyres on my 2003 Z4 with the stock tyres and rims I got from GP23 (from Z4um). Interesting to note that M's don't come with RFT's stock. The ride is so much better and handling is greatly improved. I keep an air compressor and a can of tyre sealant in the trunk in case I get stuck, but the improved performance is worth the risk of getting stuck.
If your just going to do ho hum back and forth to work,inner city driving I shouldn't think different tires front and rear would matter much.....However if you are going to do the twisties or push the car hard...I would change all 4 8)
I would be extremely cautious about mixing RFTs with non RTFs. The way the RFTs behave at the edge of their traction limits compared to non RFTs is pretty dramatic. RTFs tend to act like they are absolutely glued to the road and then give way rather abruptly instead of gradually losing traction as they approach the edge. I noted very little feedback from the RFTs at all in this process.
I had Bridgestone RE050s on my Honda S2000 and did 27 track events on those tires so was very used to how the nonRFT 050's behave and have nothing but praise for those tires out on the track and on the streets. The RFTs that came with my car had almost nothing in common with those tires.