Maybe the Corsa owner would prefer that too. However, let's just assume he wanted a car that has performance on a budget. If that's the case the Corsa beats the Z4 hands down.
It's not about what cars better and tbh comments like the above are just sour grapes. The guy's built a cheap fast car which he enjoys. Doesn't matter if it's not as good as the Z4. What matters is it fills the critera set by the owner.
There is no sour grapes what so ever, when I had my Fiat Coupe I was done by one of those Toyota little hatchback imports with the 1.3 turbo in it that was heavily modified and stripped, fair play to the guy, he happned to go to the same carpark I was going to the cinema for with his Chav friends, we had a chat, no sour grapes at all.
Doesn't mean stating I would rather be in my Coupe is because I had sour grapes, its just a fact.