Given the amount of action the insurance assessors are having with Morgan at the moment sticking 255 35 on the OZs instead of 225 40 your asking for trouble its a conformity issue. Some companies even check tyre pressures if your involved in an accident. Your choice of course I will stick with Chris Lowrance's recommendations........Em
As long as you declare it you should be fine, to be honest though unless you are being silly they are a correct fitment tyre for the wheel size so they would have a job to prove it was the primary factor if you didn't tell them.
Although Chris Lawrence did a great job every video I have seen of an S1 being driven in anger it is flailing wildly in over steer, so I believe fundamentally there is a balance issue when pushing on - and it makes sense the later cars have staggerd fitment as a result.