MACK said:
How do spring and dampers effect tyre inflation? The spring and dampers have no bearing on how inflated/under inflated a tyre will be at different pressures, that's down to the weight of the vehicle.
The easy answer to this is look at motor racing, suspension travel and spring rate in F1 are small and stiff respectively, the tyre pretty much does most of the work of the suspension. If they go to 18" wheels the suspension will have to be modified accordingly, because they work together and it is no different in road cars, except most of the work is done by the suspension, but the work hand in hand, change one and effect both. Yes the weight of the vehicle has a bearing on tyre pressures and suspension damper settings, however this is designed in at the beginning as are the variances in weight due to passengers and luggage if you change from run flats to non run flats that is no longer true.
MACK said:
Heres my argument/point of view (no doubt to be shot down in flames).
Runflat tyres have sidewalls that are not designed to flex to anything like the same extent as non runflats ones, otherwise they wouldn't support the weight of a vehicle with a puncture.
Yes and no, they are designed to be able to support the vehicle for a limited period of time without any pressure, this is because the sidewall has both sufficient strength and resistance to fatigue (controlling over temperature when flat), this allows prolonged use without pressure compared against an ordinary tyre. The flip side to this is that it is much harder for it to deform and therefore less deformation equates to less work which equates to less temperature.
MACK said:
The rigid sidewalls of runflat tyres sidewalls would flex too much at lower pressures which would damage them over time. This is why your advised to drive no more than 50 miles on a flat runflat or replacement is necessary. Hence higher pressures are required to reduce this. But this level of flex is acceptable in none run flats with there more flexible sidewalls, hence why you can use lower pressures as long as the tyre isn't under inflated on the vehicle.
I'm not sure I follow your logic here, all tyres need sufficient pressure to operate correctly, run flats can operate for at least 50 miles without pressure, where as conventional tyres cannot go 50 yards. If you filled both types of tyre to 50% of their normal operating pressure I would guarantee the non-run flat would fail first because of the superior fatigue resistance of the run flats. It's that extra flexing that causes more energy to be converted into heat, and it's the heat that causes the tyre to ultimately degrade and fail.
MACK said:
Suspension wise, its not like none M E85/6's are soft in the spring/dampener department either!
Perhaps not, but they are designed that way and designed that way to be used with run flats. Trying to compare pressures against a car that was designed to use conventional tyres is fundamentally flawed
At the end of the day you have to account for the difference in wall stiffness with conventional tyres and the only way to do that is increase pressures. As many have stated they prefer lower pressures because it will give a more comfortable ride at the cost of steering feel and turn in.
I do not disagree with it, just the logic you used getting to it, no offence intended
I have just picked up my new blue 2.5 Si sport, loving a proper naturally aspirated engine having come from a TT and constantly having to drive it on that knife edge of boost! The Z4 needs some new tyres, and I'd welcome any input / recommendations. It currently has these runflat jobbies and I'm told that it will improve handling and ride to go non-runflat. Questions are then; does anyone have any good recommendations (money not a bottomless pit!) and can I do one axle at a time? I imagine I'll have other questions as time marches on! 
I like Goodyear F1's 