I had my 1995 Mk3 VR6 chipped, and had asked for a before and after dyno reading, which they quoted in there bumf was what they supplied.
When they handed the car back to me with a dyno plot it only showed the figures with the chip fitted. On asking for the figures before it was chipped I was just shrugged off with the fact the chip had made an improvement on the performance. To be honest the car did'nt feel any different so I personally would not bother and I had nothing in my had to prove it did.
So if you do decide to go for one then make sure you get a reading before its fitted or you could just be taken for a ride.
One thing that has made a big improvement to my VR6 is the VSR manifold in the low down torque, and the fuel economy is slightly better by 1-2 mpg over a steady 20 mile run. Im glad it did as the feeling of wasting money is not a good one especially if its a lot like the VSR was.