My experience with Falken...
Hi, I've been chasing the same issue for past two weeks with a set of Falken 512's (245 45 R17) just installed on my 2003 5 series with the sport supension... I had em balenced several times, a 4-wheel alignment, a full front end inspection with no luck. Shocks/struts are fine. I moved tires front to rear and back several times with no luck at all. I was getting pretty discourged so I decided to try another tire.
The shop where I purchased the Falkens gives a 30 day exchange perk, so I took em back and had them install a set of Pirelli's of the same size ... Ready for this? It was a night and day difference! No shimmy, no vibration at 70mph, no nothing but the road feel the 5 series is supposed to have. It was the tires all along. The shop worked with me all the way... Let me add this though... We have set of P235 45 17's Falken 512's on my wife's 3-series with no issues. So your guess is as good as mine on why some Falken's are good and other are not so good.
Hope this helps somewhat.