I did a GP4 and got about 30 coins in Party Mode before calling it a night. Probably right around an hour, maybe an hour 15 of ARMS with 2lb wrist weights. It definitely got my heart rate up and for a little over half of it I was standing up which makes a big difference in the intensity. Definitely recommend that as a big point for anyone interested in the fitness potential here. Sitting down, you will too easily keep your arms from bearing the weight subconsciously (i.e. by resting your elbows on something). Despite the added effort the time went extremely quickly, hehe.
During Hedlok, I did break into a legit sweat. It also took me 4 or 5 tries to down him whereas the last handful of GP4s I've done I got him first try, so it will definitely handicap your movements/play a bit. But placebo or otherwise, I felt extra spry in the one party match I did before bed after taking the weights off (and won it), so it might actually help train performance over time!
All in all pretty good, I think I will start to include doing a "weighted GP" as part of my requirement to graduate to the next GP level, and will often keep the weights on when just messing around in party mode for some extra burn. If you don't regularly do any cardio, it's definitely going to feel pretty legit if you play for a while standing up with wrist weights, you
will sweat. I really want to try it for a 1v100 match tonight.
edit: Even before this experiment I'd had some potential concerns about the impact of sweat on the Joycons. Just ordered these to give a try:
https://www.amazon.com/dp/B06XKT1M5K/?tag=neogaf0e-20 can't argue with the price at least.