Tx for your input guys.
It was mearly an exercise in difference between crops from a "clean" FF to a "dirty" APS-C with TC.
On one hand I thought that it would take more than a 100% crop from the FF to give the same reach.
On the other hand I was convinced that the optical APS-C solution would be far more better than the FF crop.
So this was not a scientific exercise but just a demonstration of proving me wrong in my first assumption.
Just wanting to share my findings, nothing else.