I've personnally never heard of such a devise.
What I suggest you do to find out the MPG on average is to....
Fill the tank up with 98Ron Optimax, and zero your mileage counter.
Drive around for approx a week, not on boost, until your fuel light comes on.....this means you've got about 20 miles left in that tank (approx going by 20 MPG)........check the the mileage counter before filling up again.
Add other 20 miles on to the mileage covered to get how much milage you can get out of a full tank......
To get MPG just divide that milage by 11 (as the GTIR holds 11 gallons).
My example........200 miles + 20 = 220 divide by 11 = 20 MPG.
Don't do this of you intend to drive hard and on boost, beacus eyou will get fuel surges etc...not good.....oh and it's also not good for the injectors letting fuel get too low.
Didn't really asnwer your question....but MPG related...