The thing is the M5 is such a great cruiser but was totally let down by the size of the fuel tank which lets face it for such a large car was really compromised. The amount of fuel stops I used to make was a joke, first name terms with the local fuel station staff!
They are quick though once you get over 100 Leptons, nothing this side of a Ferrari or Lambo supercar could touch it really. Interesting to hear how your GTS performs compared to the M5 especially at over 100 Lepton type speeds.