The torsional stiffness of a solid circular bar is proportional to it's diameter to the fourth power accordingly a 30 mm bar will be 30% stiffer than the 28 mm bar. Based on that I would go with the 500E bar which seems about perfect.

Since the subject of 400E's and 500E's has come up I'd like to ask what people's opinion are of the differences. I drove a 500E which is for sale and has 145k miles on it and found it to be the perfect car to drive in terms of the feel fo the chassis. It cornered flat as mentioned above but road so smooth and felt super solid. Given that statement how much would I lose by going with a 400E and getting the suspension parts mentioned above. The obvious advantage would be lower cost due to the greater availablity and better fuel mileage. I would gladly trade some of the power for better mileage but as I mentioned I love the way the chassis felt on the 500E particularly the sense of solidity.