Hey Welcome to the forum. Yes, they are tuneable to a point. depending on what you want to do. Some basic bolt on stuff: Intake, exhaust, chip would be a good easy start. If you have the time and money you can go F/I.
Here try these: Bavauto.com, pelicanparts.com, bmpdesign.com, turnermotorsport.com.
|AA stage 1 software| UUC light flywheel and clutch| UUC EVO 3 ssk | K&N Intake | Ebay Headers | BMWCCA member#407674 Founding member of the hit 145 mph in my almost stock 325i BMW club.