D2 Delta PX - GPS signal at high altitude

Good day all,
I am about to purchase a D2 Delta PX. However, I have a doubt about the GPS signal.
I fly business jets at high altitude, between 30000 ft and 45000 ft (mostly between 41000 ft to 45000 ft)
How is the GPS signal at high altitude? I cannot find any information on it and want to be sure this watch is not only designed for light aircrafts at low altitude.
If the GPS is only reliable at low altitude, it will be useless for me. Of course it would not be my primary source of navigation but it would be boring using such watch with a non functioning GPS in flight at high altitude.
I am really interested in your experience and look forward to getting your feedback on this!!!

Second question:
What is your opinion, is it simply a toy or an interesting tool for professional pilots?
Thank you in advance !