based on the little pop-up spoiler i can see on the back of the porsche, it may have been a cayman, which is sort of a hardtop boxster with a few other changes...
as for the yaris - did he/she honk at you or just have to brake? that's another example of why i hate traffic circles. they only work if everyone knows where they're going and is paying attention. which very few people do.
You could be right about the Caymen. The Yaris driver didn't honk, I think they realised they were in the wrong. I only braked because my passenger yelled "look out" down my ear. He over re-acted.