Hi there, just so happens not only have I driven a Ferrari 430, but I drove it back to back with a my own Bilstein Turbo AND a stock Turbo (as a "calibrating" or reference comparison for my own mod; there were also a Lambo Gallardo and Ford GT).
While comparison between the cars' handling is much more complicated than just stiffness (weight, mid engine vs. rear engine, etc.), but if stiffness is the criteria, based on my experience there is something very wrong with the installation and/or alignment of your car. A Bilstein Turbo, weight and rear engine disadvantage aside, and not looking as pretty aside , yields surprisingly little to the F430 as far as suspension feel, chassis & road feedback are concerned. With respect to stiffness, my Bilstein Turbo, granted with Tarett drop links, feels a little, very slightly so, stiffer than the F430 that I drove, with both F430 and Bilstein Turbo quite a bit stiffer, more precise, and more agile than stock Turbo in Normal mode.

I don't know if I could help, but there are many pro's lurking here, and if I can't maybe they could. But the first thing is you are going to have to find out from the shop the following info and post them here:

1. The COMPLETE alignment specs (front & rear toe, camber, caster, etc.). They should be able to print out a sheet of all the info.
2. The ride height. How much did they lower? Based on my web research you should be ok until 25mm but going beyond that could be problematic.
3. Your current tire pressure.

Keep in mind both the Turbo and Bilstein are designed mainly for "street with some track time." Used as such the combination is among the very best, or even the best (no proof otherwise). But... it you are trying to create a dedicated track car, then you might be better off with a different car, or a different coilover with much stiffer spring. But back to the point, if the F430 is your criteria, then the Bilstein should not be so drastically different.

Lastly, I don't think this is the reason for your car's problem, but regarding the drop links (the connection to the sway bar), Bilstein does come with 2 replacement front drop links, but these also use plastic bushing. I replaced all front and rear drop links with Tarett's instead, with Teflon lined rod end (heim joint) and did find the car to feel less "rubbery/bouncy," more planted, afterwards.
http://www.tarett.com/items/996-997-...lnk-detail.htm