The main difference in this Z4 v IS300 argument is perception.

If you drive a Z4, the car is pushing your body forward. All of the air around you stays in the same place so it creates a backward forc.. air resistance (this is assuming he drove with the top down... if not, ignore this post). Plus, the Z4 is going to have a lot more noise in it than an IS300.

If you drive an IS300, the car pushes you forward along with everything else in the cabin. Also, since your confined in this cacoon, it will be a lot quieter. You feel no air resistance, so a "push" effect isn't that great, and you hear less noise, making the IS300's similar performance seem dismal.

Perception is what it's all about. Car & Driver pointed this out in their mid-size compairson. The Accord V6 is faster than a 6s, but the 6's is louder and the suspension is firmer, and they all percieved it to be faster.