Dude... door dents... I only drove my 2010 535 13K in 2.5 years, yet that car had more scrapes, scratches, and little dents than any car I've ever owned... and it was always while I was away from the car. I feel your pain. fortunately, I know a good scuff, bump, scrape artist who made me like new for ok price.
anyway... regarding your original 4K backfire observation:
I haven't noticed backfire, but I have had a stutter at high RPMs in the past couple of months with the JB4. Particularly if I'm going up hill, steadily pushing the accelerator, then goose it just a tiny bit around 3.5K, and once or twice it stumbled, smoothed out, then a big surge. This sounds a little different than yours, but may stem from same source.
To your point about Hartge.... I have to say, I sorta wish I'd shelled out the extra bux for the Hartge. I don't want to bash Burger, particularly when I don't know the 100% facts behind my car's behavior, but the JB4, while occasionally pretty cool... I dunno, something's not quite right, and the manner in which Burger puts out info and handles support is a little bit too much like amateur hour for my liking. (C'mon... firmware updates and docs via posts on the fricken "N54 Tech" forum? Seriously? Some things you have to actually troll through the damn forum to find, not even linked off their main page?
If I could have found more people with experience with Hartge, I probably would have... but I guess most people don't like being the lab mouse with their 60K+ cars
<EDIT> I should add, I'm not necessarily griping about the behavior of the JB4 for F10, because I knew the deal when I bought it (it was "beta", they don't have ready access to an F10). My problem is with how they communicate info and the haphazard way all of their instructions/etc. come across. They're very responsive to email (hell, I just got a response on Sunday before New Years within 10 minutes). But still... there's a lack of continuity that gives me pause.
I might be blindly biased by perception of Hartge's price point, but my gut suggests they're more... "together" than Burger. </EDIT>