Yep, S1 and S2 bonnets are interchangeable. Both suffer from getting wind under the ront edge. On the S1 this was countered to some extent first with a couple of small upstands on the shut panel above the headlamps and also by the alloy edge trim on the V8. I believe S2's don't have the upstands, but this doesn't stop the S2 bonnet being used on an S1 that does have them. If you are prepared to mod your car then a rubber roll along the shut panel to seal against the bonnet is a very good idea!
Chris