Hi,
I assume you are talking about a std mk5 gti ? How many miles on the car? When was it last serviced?
there are some basic healthchecks that should always been done on these cars standard or before mapping, as there are known issues that can effect the running
e.g.
oil and filter change (good practice anyway)
diverter valve (often fails giving a boost leak) , swap out for current version
PCV valve (often fails giving running issues)
plugs
coilpacks (misfires)
fuel filter
Giving your car the once over in terms of TFSI knowledge would be a real plus.
If your garage has jumped straight to turbo turbine play without knowing or checking the above items (and more) then Id suggest they dont know enough about these cars to be mapping them...
Where do you live?
Can you get your money back and book in with RTech ? or a reputable VW specialist near you
Get the basics right first, then you know you have a sound car to map onto.
hth
John