As far as the 1.8T, there has been know signs of loss fuel mileage, at the same time most people who are using this valve are more on the side of performance rather and economy. According to all the "experts" (A team member of John Force racing, 2 ASE Master techs, Owner of Dynocomp) that I have spoken to the only way they know of A/F being altered is when there is a vacuum leak in the bov itself, or in the engine. That being said, that are always two camps on any subject. Until someone can explain way fuel trim would be affected I have to stay on the side of the people that teach me.
A 50-50 valve will not effect turbo lag too much. I like the 50-50 due to the fact that is allow a cooler operating temp for the turbo. I am not saying that anyone has to buy one. But the techs I work/go to school with and I like the valves.
I am getting mine on the 21st should be installed the following weekend. I will keep track and report if my mpg change as well as my power loss/gain. The 2.0T is still so new in the states I guess there is only one way to find out.