[vachss]

For the street I prefer relatively soft springs and relatively stiff swaybars and have had good luck with this approach in other vehicles over the years. That said there are some potential downsides to this.

First. This set up makes your swaybars carry a disproportionate share of the suspension loads. If your mounting points are weak this can lead to metal fatigue and eventual failure at the brackets/tabs. This was a problem on the E36's and Z3's and led some to reinforce their swaybar mount tabs in the rear. The Z4 is beefier and I have not heard of anyone needing to do this on our cars, but it would be wise to keep an eye on these areas and periodically check for signs of fatigue.

Second. By adding swaybars you are stiffening up the suspension. Doing this without also changing the shocks means the suspension is now a bit underdamped and can be too bouncy. I haven't found a problem with this in my non-Sport suspension Z4, but the shocks are still new. It is possible (even probable) that I will find that as the shocks wear that I will need to replace/upgrade sooner than would otherwise have been the case.

Third. Every time you set up a modified suspension you alter the driving dynamics of the vehicle. Sometimes when you mess around with different bars you can end up with odd behavior like increased bump steer. I haven't had a problem with this (with just a larger front bar so far), but I don't know if this has been completely explored on the Z4 yet. I rather doubt that any huge bizarre handling anomalies will result from just sticking on a pair of bigger bars, but it's only fair to mention this as a possibility.

PS. to answer the OPs question about speed bumps: If you hit a speedbump squarely with both wheels at the same time the swaybar won't change your car's response at all (it just rotates freely in its bushings) and will feel the same as with a stock bar. The only time a swaybar will make the handling jarring is when you hit something with one side and not the other - like a pothole.