Is the front sway bar not 28mm? I was pretty sure the Eibach one was 28 similar to the Dinan bar. The stock E39 M5 front bar is 27mm, so if somehow this one is actually 27mm, definitely don't bother installing it (which is a pia btw since you have to lower the front subframe).
I have the rear bar as well, great upgrade at the time when they were a fraction of the Dinan bar cost. They don't list M5 fitment, but the rear 540 fitment is what I ordered and it fits. A few years back people were ordering them online from auto zone or advanced, forgot which one, but they could be had with a coupon passed around here for under $100. Not sure what they go for now. I wouldn't bother with the front unless you're already in there for something else with the subframe dropped. The rear is an easy install and helps dial out some understeer. Enjoy.