Hi Alessandro, Hi to all my Italian friends!
I dont think it makes a difference with the thumbs. My Viper also has the turbine looking like the turbine is angled up at the front. We measured the same with Duncan's model. No trim or pitch change. The Intake to the turbine angle is not straight but the turbine to tailpipe is level.
I am sure there is no problem to angle it either with Shims, but non of my Vipers have ever had a shim.
Good luck.
Regards Al