Is USB 2.0 even fast enough to stream a 1080p MKV?
Yes. I have no trouble playing back full-size MakeMKV bluray rips that are on an external USB "green" drive or on an AirDisk drive (connected via USB into an Airport Extreme basestation.) You'll run into trouble if you have a wireless leg anywhere, so try to have everything connected via ethernet, even if everything isn't gigabit you still should be OK.
I guess I could buy a bunch more USB 2.0 2TB external drives when they go on sale but I'd like a cleaner solution in regards to cable management.
Well, the nice thing about a NAS, or a bunch of USB drives plugged into an AirPort Extreme, is that you can put this anywhere because it is networked--that means out of sight if you choose--so cable management, size, even noise doesn't have to be such a high priority. Personally, I think you can get a lot of mileage out of 2TB external drives--connect a USB hub into your Airport and then plug in several inexpensive 2TB drives in enclosures and have them all mount on your Mac mini desktop.
Also, there are some multi-bay JBOD single cable solutions that work just fine for media storage--there's one USB cable--JBOD as in "just a bunch of drives"--the way most people use these enclosures is to keep each drive as a separate volume. I've seen these as low as $125 for the 4 drive Sans Digital SATA to USB model--pick up four 2TB drives, about $280--that'll give you 8TB of storage for just over $400 and a single cable. So what you want is possible on a budget. I have several dual-bay enclosures connected over firewire, USB and AirDisk that all take the same type of drive tray--and can easily swap drives in and out at will.
All the NAS solutions seem to be 2 drives max and get really expensive really fast.
There are MANY four to five drive bay NAS options...Netgear, Synology, QNAP, etc...these are NAS over gigabit ethernet not a pseudo-NAS a la Drobo, so look a little more, you'll find them. Folks here at AVS have used the 4 bay ReadyNAS for years (before Netgear bought out ReadyNAS.) And a good SOHO NAS will cost you, but expense is usually relative for the functionality and reliability you get. There are a lot of threads here at AVS about various NAS, Synology seems to be the most highly regarded "budget" alternative amongst the AVS faithful.
You should decide if you just need a lot of storage or whether you want storage plus redundancy, plus the bells and whistles of a true NAS, things like RAID 5, on the fly disk expansion, the ability to serve up multiple video files simultaneously, etc.
The only decent looking FW800 solution I see seems to be the OWC one but dang that is a big pill to swallow, even without drives in it.
Again, you perhaps have to adjust your price expectations, several hundred (say $400+) for a good diskless NAS enclosure isn't usurious. If that's too expensive all upfront, then perhaps adjust your hardware expectation, making due with a good multi-drive bay DAS enclosure that isn't a NAS, per se. Me, I'm kinda like you, I was tempted by the perks of a 4 bay ReadyNAS all those years ago but RAID 5 redundancy (i.e. if one disk fails you won't lose any data) wasn't worth the price you'd have to pay to get it. (And I never trusted the Drobo.) So I just made due with lots of drives and practiced good backups--as drive prices fell I swapped 1TB drives in for 500GBs, then 2TB in for those 1TBs. I have several nice, aluminum dual-bay external enclosures with FW400 and 800 and very good fans (to keep the drives cool) that were not expensive--one by Stardom and a similar one by Sans Digital. So less exensive options are out there, you usually have to dig a bit, though.
How are you Mac Mini guys handling your storage? It seems like at some point with the size of Blu-Ray rips its easier to just either get off the couch and put in the discs yourself or bite the bullet and get some sort of rackmount storage.
It's nice to backup all of your optical media, even if you don't try to have every bit of it accessible to you at all times.
And, in my opinion the size of bluray rips really is irrelevant--you can fit roughly the same number of big bluray rips on a drive for a given dollar expenditure today as you could fit SD dvd movie rips on a drive for that given dollar expenditure 5 years ago...in other words drives are bigger and cheaper today so should you really care that your rips are bigger if you're also benefitting from better quality? You have to ask yourself is the time it takes to rip and store your movie collection once worth it--bluray, standard dvd, doesn't much matter--is the convenience of having everything at your fingertips at the touch of a button anywhere in your house worth it--bluray, standard def dvd, won't really matter--your value equation is still roughly the same. Just a few years ago people were ripping standard dvds to four 250GB or 500GB drives in a ReadyNAS--now it's high def video to four 2TB drives in a NAS.
Also, just because you rip a bluray doesn't mean you have to keep it--you could transcode it to a more manageable file size yet retain great quality.
And, even if you shell out for a pricy NAS you still have to have backups of your audio and video media on other hard drives so you don't have to go through the ripping and tagging process again.