Is it best to use a BMW dealership for Dinan installations or can someone else do it? e.g. Dinan exhausts, chips, etc.
I used a BMW dealership for my 330 when I had it simply because it made sense to me. I had a choice of a BMW dealership that was a Dinan distributor or a local independent shop that carried Dinan (and other stuff). Whatever you choose, I'd make sure they are true Dinan distributer (check out their website www.dinancars.com). As I'm sure you know, working on a M5 is no trivial matter. My .02
Personally, I think it makes sense to use the dealership. In the unlikely event that you ever have a warranty issue, you won't have to worry about BMW saying it's Dinan's fault and Dinan saying it's BMW's fault. This way, the dealer can fight with themselves over it.
I had the dealer do all my Dinan mods to my M5....if you dont want to use a BMW dealer you can use an authorized Dinan shop....like in NY we have Motor Werks West in New Jersey...in fact they have done 2 Dinan E39 M5 S3 conversions. Regards S