4.10 Gears and coustom tune ??

Im getting this done monday and was wondering how much of a gain ill be getting. Its a tune he's doing with a Xcal. 2. I also was wondering how much time this will take off my 1/4. My car is an auto 04 GT and is stock except for these things. Also I heard it lowers your over all top speed but how though?

Thanks
RJ