The high power 2.4GHz with high gain antenna provides extremely high range in case of clear line of sight. However a robotpilot is very important to survive a drop in tracking.
The Thomas system has high range without sharp directional antenna too, and penetration is better (trees etc). Of course a high gain antenna improves range further. The bottleneck is more like the video downlink either with the above 2.4GHz or Thomas system.
When the range is really high, Fresnel zone is important. Clear line of sight is not enough, better if ground (or other obstuctions) are as far from the transmission line as possible. You get the best results if your base station is on a hilltop.
Can and should i use a Scherrer UHF LRS RC System for extreme long range flight ?
It think not because i think it will skrull with the FAAST codification of Futaba´s system.
The Thomas system needs ppm input, it's independent of the Fasst system.