My experience has led me to conclude that the semi-horror stories about changing over to non-RFTs are overblown. Tires and rims were the first mods I did. (I went with Michelin PSS.) The increase in comfort, decrease in NVH, and increase in stickiness FAR outweighed the slight increase in sponginess and very slight decrease in steering response. That said, from the factory these cars have a soft suspension with lots of movement and under-damped shocks. The common perception is that this was done intentionally by BMW to offset the effects of riding around on rubber bricks (the RFTs). I thoroughly hated the stock suspension with the stock tires and I still thoroughly hated it with the non-RFTs, so I re-did the suspension anyways.
Of course this is personal thing and you will have to decide what's important to you. If you're happy with the suspension now, chances are you'll be happy with it after the change to non-RFTs. Just stay away from soft-sidewalled tires and you'll be fine. Either Bridestone ss-04s or Michelin PSSs should work nicely for you.