I am installing sun-touch radient flooring in my bathroom , to help with the radient flooring I would like to install insulation to assist. My layers would be as follows (existing concrete, add rigid foam - 1/2">self leveling concrete> heating flooring>mortor>tile. My question is that i am wondering if the rigid foam will hold the weight once i bolt the toilet and have someone sit on it or if i should be worried about the tiles cracking?
The foam will only cause problems, just install it properly as indicated by the manufacturer. You will not gain anything by adding a layer of foam as the system is designed for use without the need for additional layers of various materials that will only serve to compromise the installation in the long run. Just install the system exactly how they want you to, you won't void your warranty, it'll last a very long time, and your feet will be good and toasty for years to come.