+20 is GTR offset but because its a 7.5" wide wheel it may be OK. U need to use the offset calculator and stick in ur current stock wheel specs in and the new one and see how much further out it will sit.
+40 offset will definately NOT sit flush with ur guards...it will look pretty much like what ur stock wheels are now (inside the guards).
For a GTST, optimal offsets are between 8" +30 or 8.5 +35 for them to sit almost flush with the guards without any guard mods.
If you are importing wheels from Japan, don't bother with 7.5", get 8 or 8.5 at the fronts and 9 or 9.5 at the rear.