IMHO, YMMV. I don't think it is so much about the light levels affecting plants, as it is how the light is affecting the overall grow environment. Which, in turn, is what does the affecting. So..yeah it affects the plants, but not directly so. Sort of a chain reaction event. If that makes sense.
If I am trying to burn away at 1000 PPFD during the summer months here, even indoors in the tent, then that will cause me all kinds of heat and humidity issues in my environment. When my AC kicks off, I've seen temps go from 80F to 90F, I've even seen right at 100F before.
Obviously those temps above = 90F+ cause all kinds of related issues. So I have to really dial it down PPFD-wise. Maybe I run at 600-700PPFD during the hottest part of the day, or whatever.
And this time of year it's a cakewalk, for the most part, for me to sit right at my ideal 80F/60RH range for the entire run(s) despite how hard I crank the lights. Don't even need to run my humidifier.
As long as my environment is solid, I find I can do pretty much anything with the lights. And really get into my mad scientist experimentation.
I personally do not think your "issues" are light related. You're sorta answering your own questions in a way. You're feeding heavier than normal, etc etc. Which is something that I personally think is GREAT. All about that as you know. You gotta experiment and do the old "Let's see what happens!" thing to learn and such. Whenever I play with nute levels, I always play with the base nutes first.
As I have mentioned in the past, I have ran my auto plants on Floraflex at beyond photo levels and seen no real issues. I may not NEED to do it, but I have, and I have learned what I can and should not do right? Even if it is for no other reason than simply wasting nutes. Autos can take way more than most people think, and can be pushed much further. But you find a point of diminishing returns. Like any other plant.
So, I'm not familiar with your nutes. Are you adding additives to the base nutes? Because this is where I have run into problems. Sounds to me like you are seeing a mag lockout. You have to figure out why. When I have seen mag lockout in my tent, it has only been when I play with Recharge. It's recharging "something" that my floraflex doesn't like. Maybe making my calcium more available? Whatever. I stopped using it and my problems stopped. You might be seeing a similar issue with something you are doing. The cal-mag though, it's not going to help the spotting, and may actually make it worse, even though you are using small amounts. Get a bag of straight Epsom salts.
When I decide to bump up my nute levels to see how hard I can push, on my quest for a 2lb auto, I no longer add anything to my nutes. Maybe a little silica here and there. That way I can gauge how my plants are reacting, what they want, what they don't want. etc. And then, if I decide I want to play with an additive I will.
