boost creep is when you set the boost for lets say 15 psi and the car some times starts going higer than that or is inconsistant. had that problem in my car and fixed it with a 38mm external wastegate.
im sure there are better ways to explain this but those are the basics.

as far as your t25 i did all the mods that goat said to my car and never had a problem it did creep in third gear though it would hit 19psi but then fall off to 15 again. Its fun for a little while but then you start wanting more so be smart and save your money for a 16g.