For the purists it is worth noting that the factory never called these cars a Vixen 2500 - it was just a 2500 and when they switched the chassis, it became a 2500 M. The Vixen 2500 nomenclature was sponsored by the US dealers only, IIRC.
The M chassis was reputedly a big step up from the non-M chassis, but I raced a TVR with the early chassis for several years and it was very good indeed as well as significantly lighter than the M chassis cars.
It was obvious that the US was a pretty big factor for TVR, though. They never bothered to offer a Triumph PI engine in the 2500/2500 M, just the federalized 2500 carbbed version, which is a real shame as the carbbed engine had only 104 bhp vs. the injected version used in home market TR-6s which had an ouput of 150 bhp - a world apart as far as performance is concerned.