nice grow i like your setup.
Overall i like what your doin i have just a few small suggestions.
1. I noticed at the very beginning that all your lights are "inline" with 1 fan pulling through them this is a nice simple way to go, but every time your duct angles you lose CFM and with all those 180 degree bends thats serious strain on your fan i was gonna mention if your temps were high but by the time i got to the end of the thread you had your temps under control and a new fan already purchased. Another way to reduce temps (by cooling the bulbs more efficiently) would be to have some Y splitters at the top of your cooltubes so that they are not running inline this way the heat of one bulb wont heat up the next one inline. You'd also have to move your air filter to the outside of the room and blow air through it instead of suck because rigging a plenum from that to the fresh air for your lights would be a pain.
2. Even if the plants seem to be growing "even better" when you bump the ppms that high, it might just be cause they're getting more established and would be growing that much anyways. The higher you force your ppms, even early in flower, the more you have to flush at the end. If you give them 1200-1500 they'll probably grow just as fast and it will be much easier to flush and get a fully tasty product. I used to run my bloom nutes at 1500-1900 in hydro, now i do around 750-950MAX but thats just me.
thats all i could think of these are minor points your shits gonna yield no matter what good luck!