Sounds like you've bought the wrong car. The 6 is a long distance cruiser, not a race car. If you want rigid, go coilovers. It will change the way you drive you're car, and not all in a good way. Gutters, driveways, potholes, speed bumps, you're always looking out for them, it gets tiring. Yes the handling is awesome, but my car has lost some of its appeal because the ride is so hard.
Personally I'm more than happy with the coil-overs set-up, its true what Yeldarb says you gotta watch out for some of the hazards but I'm more than happy to put up since I get back a lot more in return when i want to.
I would pick the coil-overs over the stock set-up any day of the week, however i would pick the stock set-up over just springs.
You will not achieve a stiffer ride with the Eibach/B8 combo. You'll get a lower ride height with approx. the same suspension feel as stock. Coilovers are you other option.
The 6 is a GT car and the AWD makes the driving proposition different to most other cars on the road. I've driven S4's, M3's, Liberty GT's and the 6 drives very differently to all these, particularly in spirited driving situations.
I have to ask though, what car have you come from, and would are you trying to achieve with altering the suspension setup?
I just want less body roll because there's so much... I'm not trying to turn it into a track car with super stiff and hard suspension so theres absolutely no body roll. There was less body roll on the stock springs..
Hey I am running the Eibach's , Bilstien B8's , Whiteline rear sway bar and 19 x 8 rims. Front bar is the standard.
be warned the rear bar is a bitch to fit - on mine they had to drop the rear diff to get it in
The ride is stiff enough and the handling is fine.
different car, but i just installed the pro-kit/b8 combo on the gen1 3 mps and its a whole different world. admittedly, the stock shocks were gone and I never drove the car new. Although I got it at only 16000km, they were shit then and only got worse
controlling body roll through stiffer suspension is not ideal. as others have mentioned sway bars will provide the most difference. this will leave the suspension to do the work they are suppose to - keep the tyres on the ground (bound and rebound)
for the street, i dont think coilovers are needed. unless you have top end coilovers AND have the knowledge on how to tune them, the ride is generally not desireable for the street.