Okay, other fans arrived! Now I can replace the stock fans with same-size (92mm) using the native, modular fan cages...but these blow a lot
more air. The two delta fans in the front will help blow through the first two heatsinks as well as move more air around the case.
I was also able to add a small fan to the rear of the case, to help out the fan-deficient HSF (because of necessary GPU; see OP). I saw immediate, collateral results: the small mobo heatsink directly in front of it went from blisteringly-hot to cool. Sweet!
I can also, incidentally, replace some of the HSFs, from the Noctuas I have. This brought me some trouble -- I got a much more powerful fan (12W, 38mm thick). I plugged it in to see if it functioned...and it flew violently at me. So, any mounting solution had to be pretty bulletproof...
I initially tried using a shroud on the intake of it, and mounting it between heatsink 3 and 4. The shroud for intake didn't work, unfortunately, as the retention clips weren't sized for that. I then tried [successfully] mounting the shroud at the exhaust...but the Tornado didn't pull in as much air as I'd hoped, leaving heatsink 3 too hot.
So, at risk of burning them both out, I placed the Noctua as pull on heatsink 3, and left the Tornado as push (with shroud, for retention clip usage). I felt like returning the other noctua to pull on HSF 4 might be a poor choice, adding to the burning-out...but...unsure.
I foolishly tried moving a fan while Summermute was on (hey, gotta fold them proteins...) and this flew at my face.
Lesson learned, Tornado.
(and yes, now it's unbalanced as hell and vibrates menacingly. I...refuse to try gluing it back on, so if it ends up being a problem, it'll have to wait until next loan disbursement.
And, this is the final product! Added a rear case fan, replaced one front case fan, replaced two HSF with one 38mm, minor cleaning. The BitFenix 200mm fan won't be there for long, as I'll be replacing the side panel when I move the server (to the window, I think: it'll be louder, but cooler; who needs quiet when studying? The screams of pathogens should drown out any loud deltas anyway)
Non-ideal situation at the rear two heatsinks, but...I'm out of ideas and both heatsinks are now finally only warm to the touch (versus very hot), which is
in my book.
All of this thermal stuff means...OC'ing time!