I agree with those who feel that the Zed is deceptive.
Yes, the Caterhams I had (and the Westfield) felt madly quick at the time but that is partly down to the fact you are 4 inches off the ground and the amount of noise (particularly in the Crossflows).
Maybe for the first bit, if it was dry and if you got everything right, they were quicker off the mark. However, it often surprises me, with the Zed, what the speedo says, which I find difficult to relate to what my senses are telling me.
The only thing I miss, in truth, is that assault on the senses - but that can wear a bit think after a while. They thing I miss (without qualification) is the way they take the bends (and roundabouts) which has to be experienced to get it. Overall, though, given a mix of conditions, I am happy that I made the right choice.