Full disclosure: I don't have an E70. I have had three sets of run flat tires. RFTs tend to generate strong emotions, usually against.

IMO, early run flats tended to be very harsh. Each generation has improved, and recent RFTs can be very close to non-RFTs in comfort.

Manufacturers have also tuned suspensions in recent years, to reduce harshness. All that means is that harshness correlates to vibrations at a certain frequency, and they can tune the suspension to deal better with those specific frequencies.

The X5, particularly with larger tire sizes, already has very heavy wheels and tires. RFTs make it worse. It is also a heavy vehicle, and it isn't going to feel exactly the same as your E53.

Tire pressure can make as much of a difference as tire construction. If they seem harsh, make sure they aren't over inflated.

RFTs can shred, exactly the same as regular tires, if owners drive on them for long distances at high speeds when they are deflated. If owners ignore the low tire pressure light on the dashboard, they will end up on a flatdeck.

RFTs do not necessarily have harder rubber compounds, they have stiffer sidewalls. You can get RFT with hard or soft rubber compounds. I have summer performance tires on the 535, and a set of winter tires that perform very well in cold temperatures, and both are DSST (Dunlop's version of RFT).

RFTs are not all the same. Some are harsher, some are less harsh. Same as some tires corner better, and some tires wear more quickly. It is a gross generalization to say that all RFTs have any particular characteristic when compared to non RFTs.

RFTs have stiffer sidewalls, and that makes turn-in crisper. That is the same reason people buy larger tires with lower profiles, to get crisper turn-in.

I understand that some people don't like RFTs, and replace them. I was impressed enough with my Dunlop DSST tires that I purchased the same tires for winter, in a different tread pattern and rubber compound.

If you want to swap them, just decide if you are going to carry an inflation kit, or a cellphone.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue