I can relay some opinions for you;
1) There does seem to be a reasonable incidence of top end problems
2) Hearsay would suggest that there was a batch of dodgy finger followers, that were not marked so impossible to tell which cars they went in
3) Some dealers have suggested a camshaft replacement (too extreme profiles?)
4) There are at least 3 independent garages doing rebuilds, using their own fabricated components (as mine will have once rebuilt )
5) Mr Melling (SP6 and AJP8 designer) gave a talk in 2004 to TVR owners where he poured scorn on TVRs effort to productionise his design. The idies have various methods of returning the SP6 the Melling original
Every car is a compromise somehow, I found it best to do the homework before plunging in, setting myself a realistic expectation of the car's capabilities, reliability, costs and most importantly grin's per mile
Thanks for shedding some light on that