I can't imagine it would be any more significant than having tyres from different manufacturers on the same axle - certainly a hell of a lot better than that bloody space saver!
Lol I wouldn't do that either. Ok to get you home as I said but IMHO asking for trouble at speed then breaking hard or mid corner parhaps I'm just conservative.
One or other wheel, even on the front, sticking out a further 4mm is hardly going to throw the whole balance of the car out ? Probably more discrepency in the average TVR chassis than that anyway