Have you driven one with 380bhp, it would run rings round your C55 mate. I have a Golf MK5 with 380bhp, you wouldn't see which way it went trust me.
If I drive in=t normally 75/85 on the motorway, I see 38mpg all the time. The 6.2 N/A 63 will not do that and neither would your 55.
But are we talking about cars with mods here? Anyone can use that argument if they mod the sh1t out of their car...