I use 2011 460 cm NP X6 with 2011 Naish Grand Prix 7,0 and it works great.
Rigged 2009 Gator 7,5 with X6 460 and works perfect, better than my older Blue Line mast.
A year or 2 ago some pro riders used Severne masts in NP Race Sails.
So I don’t know how valid this mast selector is.
I would give it a try. Remove all spacers first and then rig the sail with the X9, then tune the sail with different spacers if necessary. Maybe it works fine and you safe some money.
I just had to buy a new Enigma 490 for my 2010 Overdrive, because id did not rig proper on an older Redline mast. Now it works fine.