Any particular reason we use HTD belts instead of GT2? My very shallow understanding is that GT2 is the more tooth profile and should handle higher loads a little better.
But at what cost? That’s gonna introduce more friction to solve a problem that doesn’t exist. I run 12mm belts on 12s and I weigh alot I have yet to skip a tooth on street set up.
The link you provided yourself tells you why 
From what I read you can use GT2 belts on HTD pulleys, but not HTD belts on GT2 pulleys. However, it is not recommended. There is nothing wrong with using GT2 pulley as it was a further revision of HTD to reduce backlash. You’re free to use GT2 pulleys and I bet those are what boosted boards uses becuase of the 3mm pitch design for the pulleys.
I have a gt3 belt here I want to try when my new 15mm wide pulleys get here. Have some idlers I’m going to add also.
@danielz what do you mean? the doc says 3mm gt2 is rated for more power than 5mm HTD, and the compact size would mean we could run more aggressive gear reductions without running into as much clearance issues. Or we could run narrower belts with 5mm GT2 and fit dual 6364s without having to use TB 218 or other unusual trucks. I dont see an obvious downside, other than parts availability.