6-10% is achievable from dealers, however remember even if you go via a broker, the dealer normally does the PDI and handover anyway - if the dealer is half decent they will do this professionally so you will go back to them in the future
I managed 11% from a dealer on a 3.0 SE V6 TDI including options, so clearly it can be done. I have also seen posts on here where others have achieved similar and more. I would have bought through an Internet broker if the dealer hadn't matched the offer available at the time. Might be worth trying another dealer first but provided you have read all the small print from the broker and are satisfied that there aren't any untoward risks then why not.