thx bradc,
re the pad touching all of the 294mm disc: that's a perfectly valid point .
it may be best if I wait to see if anybody with a facelift 'non VR4' fitted with the 4-stud 275mm discs and twin-pot 158mm calipers comes along to confirm that this combo was in fact used.
then again, i could just source some used twin-pot 158mm calipers with old pads and see how the pads sit against the 275mm disc and if there is pad overhang forget it as a feasible upgrade and re-sell the calipers, negating purchasing a caliper rebuild kit and expensive new pads.
have submitted an enquiry to National Brake Discs re post 19 so will be interesting to see what feedback I get.