Watt for watt the 5000-6500k spectrum will probably have better par than an 8,000k, or a 10,000k, the higher you go, the less reds and yellows you have in relation to the blues and purples. But when you have major overkill lights such as an HQI, your photosynthesis will be maxed out even at a higher Kelvin. I'd just keep it at 10,000k or under, but if you are at low light already, probably stick to the 5-6500k.yeah, for my plants i always go for something between 5,000k to 6700k, but your plants seem to love it, and the colors do look good. i may have to do a bit of experimenting....
I had the same problem with my planted biocube 14. I can't even get my hand down chamber 1 to plug the lower vent. I'd rather not block from the front due to ascetics. It's very much a pain!Good question, The most important mod is keeping the shrimp out of the back. To do this, I plugged up the lower vents with some filter floss, you could also super glue a sheet of plastic over it, from the front side or the back, or glue some mesh screening, or stainless steel mesh over it. Then, I had a stainless steel filter intake screen, that was probably 1.5" in diameter and ~7" tall, I cut it open down one side and across the bottom then butterflied the thing open. I tried using superglue gel and hot glue to attach this over the upper vent slots, I found the hot glue works best, Just make sure you get this on good because if it busts loose later on it's a pain to fix! If you can manage this you get an awesome shrimp tank!
Other mods, I grew some emersed plants in the back chambers. That living basil you can get at the grocery store does really well with it's roots wet (might have to find a creative planter to keep it in, or a pot just larger up top than a chamber), but keep the stems above water. It will send down extensive roots, which will help keep the water clean.
Also-you may want to go open top with a custom light such as LED if the temps are running hot. With that enclosed lid and all that PC lighting, it can generate a lot of heat and keep it in. Just set it up and keep an eye on it for a few days.