Your dealer is,excuse my French, full of s***. It is possible they could deny warranty coverage for an item specifically related to the front suspension, since that is where the modification has been made. But they cannot, by law, deny all warranty coverage. This issue was settled in the courts several years ago. SEMA (Specialty Equipment Manufacturers Association) was a plaintiff to a large class-action suit for this very reason. If the dealers could get away with this nonsense, they would be able to shed all warranty liability for stupid reasons, like...."you didn't have your oil changed with Ford brand motor oil, therefore your engine is not under warranty", or "those aren't the OEM wheels, therefore your transmission failure is not under warranty". The only loophole they have is if they can prove that the failure is caused by the aftermarket piece, then there is no warranty coverage. But, the onus is on them to prove it, not you to prove it didn't.
Hope this helps, Luke
And modify away, find another dealer if you have any warranty problems.