David H,

I have the following tyre on an old 1973 Porsche 911 in of course another size. It performes very well and has a reasonable price tag.

https://www.dunlop.eu/en_gb/consumer/tires/sport_bluresponse/205-55-16-91-V-3188649819256.html

I did not know you are searching for a 16 diameter tyre which means you might use stainless spoke wheels? This tyre is very good on a Porsche which means, if the predecessor of this tyre was a Porsche „N“ recommendation (they quit to test all the descendants of any brand) it should be also a very good performance tyre on a Roadster. The cars als quite similar regarding the weight, the 1973 Porsche is about 1040 Kg and therefore a little bit heavier but not too much.

Please check if my example shows the right load index for the Roadster, my search was quick...You could give it a try.

'14 4/4 graphite grey