i double tint my car.
1st layer has about 80% of IRR rejection.
2nd layer has another 85% IRR rejection.

1st layer is titanium grey. 55% VLT. kindda shiny black. then it faded after a year.
so i weighted my options either to remove or apply another layer. a new layer would only fade (again) after a while.
so i applied another layer of black tint (not shiny) with 25% VLT. much lower than the law which states a minimum of 50% VLT. gonna take my chances with the JPJ.

at first it looks so darn black. can't even see anything inside even if you stare.
then, after more than a year it faded (again).

now closing to 2 years running on double tint, my car looks like your normal-newly-tinted-passing-JPJ-requirement and it doesn't fade anymore than it is. i park my car everyday under the scorching sun so fading is normal.
talked my way out of my first JPJ roadblock giving problem with my tint, and no more after that. JPJ now allows me to pass.

never had a problem with peeling, bubbling, tearing and GPS. i don't use SmartTag. but if GPS has no problem, i'm vouching that SmartTag would also pass.
make sure there's no problem with the 1st layer tint before you go and apply the 2nd layer. and the good thing about double layering is that even if the tint fades, the IRR stays the same. much less heat through the window now.