Firstly I have done this earlier on this year, had RFT's on the back and non RFt's on the front, the reason was i put the part worn RFT's back on the rear for a track day, then left them on for a second track day a month later after which i switched back to the non RFT's, so, i've got to say all the fears that have been mentioned are totally unfounded, i ran this set up for approx 160 laps of Croft and approx 600-700 road miles in the time, the car was superb on track and it was fine on the road though there was a difference in the 'feel' of the rear of the car, from my experience i would say you have no problem at all running RFT along with non RFT.
secondly you talk about switching the whole set and opting for a 'punt on the Falken FK452's well this is the tyre i have on my 3.0i and i have to say it is simply the best tyre i have ever run and i have done all the usual on previous BMW's, PS2's, potenza's, Dunlop's, conti's. it totally inspres the chassis of my Z and it is not a 'Punt for you to get these it is a great choice and you will never go back to the over priced branded stuff again, try here for prices,
http://www.falkentyres.co.uk