I just ran a JB+ on a coupe 335i. It is easy to connect and disconnect in ~5 minutes which is what i liked so ic oudl bring my car to the dealership and take it off and not void my warranty. It was noticable power increase for sure and that was at the stock setting you could still turn it up (but risk throwing codes)
Currently Drive: 2011 S4 Prestige, Quartz Gray, Black Alcantara, Adv key, B&O, NAV + rear sensors, 19's, Carbon Fiber Trim, iPod...
Previously drove:2007 335i Coupe, Titanium Silver Metallic,Black Leather, Gray Poplar trim, Premium package, Cold weather package, Sports package, i-Drive, Rear sensors, iPod.
Previously drove: 1998 3000GT Vr-4 Red/Black TT AWD AWS