I drove both and there was virtually no difference with the top up.

I've yet to get any sense of cowl shake or instability in the roadster. It is easily as tight and sure of itself as my MX-5 was -- and it was incredibly rigid, due to its light weight and compact design.

The 370ZR is worlds better in this regard than my two BMW convertibles were (2000 MZ3 and 2001 330Cic)

Any cowl shakes when the top is "down"? The Supra that I had shook with the top down.