The greatest attribute of a Z4 is its weight. With that being said, a 2.5l is not going to give a GT a run for its money. Now a 3.0l on the other hand is quite strong. The inline 6 has some great torque and combine that with a fast reving engine in a light car and it is pretty quick. Depending on the year of GT and wether it is an auto or not, I can run a GT with my 330i and it weighs in at 3500lbs. Now imagine a car with the same exact engine weighing nearly 500lbs less and you get the picture. I know most 3.0l put down around 190rwhp, expect a little more for a sport package equipped car with tuning, cam, and exhuast, close to 200rwhp. The torque is great though, I was quite surprised to the torque of the inline 6. It nearly mirrors a V8 engine, over 190rwtq.
If they are racing auto equipped GT's I can see them rolling them, but a manual equipped GT, it would be drivers race, with the GT pulling at the end, especially anything from 99 on up. The problem I have found with most BMW drivers that drag race, they do not really "drag race". Most are impropmtu roll races that are usually in favorable rpm ranges for the BMW owner.
Good luck convincing them otherwise. Over on a BMW entusiast website it is corroded with crap races in which the cars that these people claim to race would just destroy them, but somehow it ends up being the other way. A good example, a guy claims his M3 with a dry 125 shot(crankshaft rated) put over 3 car lengths on a Camaro SS that had headers, intake, chipped, camshaft, and TNT wet kit jetted at 150. Of course I laughed at the "chipped" part, but I thought it was freakin hilariuos that a car that puts down nearly 400rwhp on motor alone with a wet kit rated at 150rwhp got beat by a car that dynoed barely 400rwhp on the juice. I pointed out that my car stock with the same wet kit put down 477rwhp and nearly 500ft lbs of torque at the wheels. I would have believed the story if it was just a cam only car.