I haven't been a lead game designer in any game project myself, but I've been part of discussions around game design decisions in small teams. I don't use video game design specific theory in what I do, but there is a lot of theory from the UX field that carries over, as the physical and digital interface design aspects of web/app/product development carry over to video game development. For the most part UX design is scoped around applying research design to find ways of measuring aspects of human psychology around the experience of a thing and present this it in form which is useful to the rest of the team. But I am not really trying to go this far with this service, I would just be the tester and use the measuring tech that I would usually use on other people on myself, so if there are any indies out there they won't have to spend a lot of money on this equipment themselves. Breaking all that material down and analyzing it would be left up to them to do.