The larger front bar results in less roll and better camber angle for the front tires, and it especially results in better transient response (i.e. slaloms in autox). I pulled the alignment pins, max'd negative camber (got -1.7 degrees), and installed the Dinan front bar (btw, unlike others' experience, there was zero preload on my stock sway bar and the Dinan bar once it was in place, so I had no need for adjustable end links).
I'm running the car in F-stock this year, tire class, with BFG Rivals. So far, I'm very impressed with the car on course. I can't actually imagine it without the front bar as turn-in rotation was fine as is. In pure steady state cornering conditions (i.e. mid-corner), I imagine there is a bit more understeer due to the bar, but it's easily manageable. Slaloms are a hoot though! Also, this thing will really put down the power on corner exit -- really impressed with the M3.
When you do the install, take off those side pods on each side first and that will allow you to put a wrench on the weird wire supports (x2 each side) for the center undertray.
2011.5 M3 sedan 6MT, Silverstone, no sunroof, no EDC, but all other options
2001 M5 LeMans Blue/Silverstone
2006 330i 6MT Red/black, ZSP, Nav, etc.
2001 Audi S4 6MT; 1996 Volvo 850R wagon