I had my Z4 remapped about 6 weeks ago at `Road Rage Tuning' in Wrotham (Kent). Basically its a couple of young fellas with a rolling road who rent an unused barn on a farm complex.
I'd made a number of enquiries at various places and knew theres not a great deal you can do with the 3.0L. All the quotes came back with a 7-9% increase in power/torque at around the £400-£500 mark. I decided to leave it. That seemed too much for too little, if you know what I mean.
Anyway, about 2 weeks later I received a promotional email from RR offering 10% off all remaps. They just happened to be the cheapest of the quotes I'd had...and I bit. Here are the increases they got out of the car:
Before: Power 243.2HP @6300RPM - Torque 225.0(ft-lbs) @2800RPM
After: Power 267.8HP @6800RPM - Torque 282.4(ft-lbs) @2050RPM
So taking into account the RPM discrepancy, I make that an increase just shy of 10%....for £360....and I can definitely feel the change. The car is just that bit sharper, more responsive. Its obviously not huge but it is noticeable and I'm more than happy with my purchase.