I know what you mean about the sport aspect. Hidden plastic is the lightest and most mobile, with no penalty as far as combat. The one season I did SCA heavy, I did it in a pseudo-16th-century-italian-inspired harness, and I sucked. I lost a solid 95% of the time (or more). Could I have been remotely competitive in 70-ish pounds of armor compared to others in 20-25 pounds? Nope, not at my athletic level. Even at peak, I would have to be considerably better to squeak out a few wins by having so much more encumbrance. I even wore cased greaves, just because I liked the look, even though lower leg isn't a valid target (mistakes happen).
Was it fun? Yep. Do I prefer WMA / living history instead of "sport" style competition? Yep. Personal preference, and those who enjoy SCA seem to really enjoy it, so more power to them - as long as there's swords involved, I won't complain.
Hidden armor, leather armor - all look appropriate on the eastern european styles, to my eye, but I don't know much of those styles either, beyond bazubands and basic helmets / cuirass (char aina).