40 amp Pool heater hooked up to 60 amp outdoor Spa Box
Double check everything, since Siemens makes several lines of breaker/panel (which are physically incompatible so they plainly won't fit), but it all makes sense to me.   


You made a canny choice of hot tub subpanel so you could swap breakers - smart economics. 
The farther upstream the GFCI protection is, the better - and you can't get more upstream than the main panel, can you? :) 
The breaker in the panel is able to protect the #6 cable and 60A subpanel.
The breaker has the correct thresholds for a hot tub (8ma and not 30ma) since it's sold as a hot tub panel.
With the subpanel fully GFCI protected, you can put plain breakers in there.  Though a typical dumb home inspector isn't going to like it.  


Be sure to put "GFCI Protected" labels on the receptacles and subpanel, I recommend making your own with a common labelmaker, as they last longer than the blue paper ones that come with GFCIs.