I'm thinking I may sort mine in that way - it's plenty quick enough for me, mind, but I know what happened when I remapped my smart roadster and the pressure went up from one to 1.5bar - that knocked an easy one and a half seconds off the 0-60 time and other mods have knocked another second and a bit off. It left the factory with 80bhp and now has 117bhp.
The XM is a motorway mile muncher rather than a hooligan's sports car and should be a little sedate but I know that the odd bit of acceleration can be exhilarating!
