Sway bars tie the lower suspension components together accross the front and the back, they affect the car oversteer and understeer. Bigger sway bars improve stability while cornering, the best setting is fully soft for the front and hard for the back.
Stock: Front: 23mm - Rear: 15mm.
Sport suspension: Front: 24mm - Rear: 18mm.
M-Technic suspension/US E34 M5 3.6: Front: 31 35 2 226 333 - 25mm Rear: 33 55 2 226 507 - 18mm
Eibach: Front 26mm - Rear: 13mm. (yes, I called them to confirm)
Racing Dynamics: Front 27mm - Rear: 19mm.
Also there were Nuerburing (BMW's spelling) rear swaybars for the M5 Touring. These were the biggest rear bars ever offered. These may be the perfect complement to the 27mm bar offered by RD as their front rear ratios are highly biased to understeer and these may improve the situation somewhat. Rear 33 55 2 227 416 - 20mm
31 35 2 227 276 $118.00 (m5t front)
31 55 2 226 334 $6.00 ea. (mounts)
33 55 2 227 005 $108.00 (m5t rear, 19mm)
33 55 2 227 006 $4.00 ea. (mounts)
33 55 2 227 416 $124.00 (N'burgring m5t rear, 20mm)
33 55 2 227 240 $4.00 ea. (mounts)
With a 20-25% parts discount offered by several BMW dealers in Roundel, and considering that rubber bushings are available from the factory (much quieter and transmit less vibration), the factory bars are a pretty decent choice without being prohibitively expensive. However, there is a time factor to getting these items as they have to ship from Germany, those ordering the Euro parts will have to be patient...