Quote Originally Posted by e34.535i.sport
The firm I'm using has a Dyno and with before/after runs with an individual remap he charges £275. I've heard from a few sources that he has good results with BMW petrol engines and obviously with an m30 there is scope for improvement.

Your absolutely right, the factory naturally de-tunes the engines slightly on purpose for several reasons (people using poor quality fuel/missing services/etc) generally not looking after it. Therefore it can be improved... but as you say advanced timing will probably require a higher octane fuel. I'm wondering whether half premium and half 'normal' fuel would be ok?
If you use lower octane fuel isnt there a risk of pre-ignition? That blows holes in piston tops and valve faces. Not too good for gas flow and terminal if you let it go on too long. Certainly wouldnt do much for performance. Sure the tuning firm would tell you what's best. They may also have some comments on the quality of cheap supermarket fuel