Lee at Invisi has specifically chosen the current thickness of his 3M to meet 3 things. Be of a strong enough adhesive to stay on the frame and not allow mud/water ingress. Be not too thick so that if the owner or future owner would like to remove it that it will come off without peeling away the paint and finally being of a reasonable weight as it tends to be applied to mid-high end mountain and road bikes.
In regards to its strength. My brother had the mother of all stacks this year on his Orange Alpine and took a rock to the top tube. At first glance it looked like he may have wrote off the invisi protecting that area and maybe even gone through to his paint. However, after a wash and a bit of muc off bike polish it was exactly as it was before the accident. He was made up!
My own personal experience is completing the Trans Provence with the invisi protecting my bike a treat when many others finished looking twice their age. I have also just got back from a week of DH Shuttling in Malaga and once again it took the hits, especially around the underside of the chainstays.
For £53 for the full kit you can't go wrong.
All the best,