Here's what he says

he's name is HerrRossi77 and has a lot of experiences with Z4M and tuning)
" Got the same installed and can only say: SICK!
Vmax Tacho's always another 300 so you have no disadvantages compared to the standard translation.
Clear advantage in terms of elasticity.
100-200 in 4th gear going slightly faster than the production 3g/4g response (please note here speedometer reading).
200kmh with the 3.91 will not be matched.
My value in 4th gear at the speed limit are as Checks Wernau measured 193.7 km / h.
Consumption of course increased minimal, especially at cruising speed because the engine turns higher, resulting as well but a logical consequence.
Moderate elasticity you really believe to have 30ps more!
This results in values that are at eye level with a GT3, ONLY in elasticity of course!"
and here's another thread from Harcy he's from Luxembourg and has it also.
"Can tell my experiences with my ex Z3MQP report that I had reduced from 3.15 to orignal 3.46er.
The difference was grandiose, a difference of day and night. The car ran well, thanks to the shorter translation. more Biting and subjective as if he had 10-15hp more. He had now in every aspect of life much more Bums. In everyday life, one could overtake easaly without even have to downshift. Also on the ring, because he just had a lot more bums, especially the output of the curves, or in different sections where you could certainly make a good couple of seconds, thanks to the diff.
I even made the comparison against an original Translated MQP, where I gave him the 5th of 120km / h to 220km / h put clearly. In the same game in 4th gear ....
The only downside was that he had a 5Gear transmission and it unsuitable for long motorway sections was.
The reason I wanted to rebuild the Zetti 6Gears with 3.91er diff.
I can recommend a shorter diff to everyone, thereby making the car much more fun and is noticeably better. The fun factor on the car increases enormously!
It was a google translation so dont read the mistakes i had not a lot of time to translate ;-)