Well, it doesn't have to have enough bandwidth to drive Gigabit Ethernet, Wi-Fi, 2x SATA and 4x USB 3.0 devices at the same time.
But would be good if it had enough bandwidth that it could drive 2x SATA or 4x USB 3.0 devices.
The PoE doesn't have to drive all those devices, as long as it could drive the bare minimum computer with just a SSD, without any USB devices, or just a USB device without the SSD.
That still would probably be too much bandwidth. Keep in mind that while ARM is significantly more complex today than it was 5 years ago, it's still simple to the point that I don't think it even has a northbridge, let alone a southbridge. Another thing to consider is USB is very CPU dependent, even when you have a proper controller for it. As far as I'm aware, there is no PCI bus, at least not in the traditional sense that you find in x86. I'm not going to pretend I know all of the details about ARM, but after owning 4 different ARM platforms myself and doing countless hours of research on other ARM platforms, I've quickly found that you have to forget everything you know about x86. It's kind of like comparing a monkey to a fish.
Anyway, I don't really understand the purpose of you wanting PoE if you don't expect it to be powering all of your devices, especially if you want wifi. Wouldn't it make more sense to have the gigabit ethernet, no wifi, and 1 small power source that can feed all of your connected devices? This is assuming the device is connected to a router. You could probably crack open a cheap 150W power supply (if you can even find one that low), stick your ARM platform and maybe an SSD on the inside, tie the green wire to a black wire, connect everything you need to, and cut off the excess wires. You might be able to remove the fan to help save space (it shouldn't get very hot in there). That way you get a tiny fully functional system. It'll be a little ugly (especially if the power connectors aren't sleeved) but at least it's all 1 unit.
If you want this for an HTPC, I'd suggest you get USB 2.0 hard drive enclosures. Those will work just fine on even the crappiest of ARM platforms. You should also look into some of the i.MX6 boards - some are known to have 1 SATA port, gigabit ethernet, openGL and GLES support, and I think 4 total USB host ports (keep in mind I said HOST ports. You want to avoid devices where every USB port is in a hub). I'd also recommend any A15 boards such as the Arndale. Also, I'm not sure if this works but you could try using a SATA splitter. Those don't work on every controller, but it might be worth looking into. Keep in mind SATA 1.0 is fast enough for the average mechanical HDD, and most SATA compatible ARM devices use SATA 2.0.
this is a lot to ask of low-power low-cost SoCs! bottom line, there's no way it can be done - not in a single processor, that is. the closest possible candidate is one of marvell's high-end "server-style" SoCs. i know that there's one that has dual GbE, and it might even have one SATA. it definitely has PCIe - most of marvell's high-end server-style SoCs do. the problem is, though, that marvell's core (Kirkwood) is basically a from-the-ground-up redesign (reimplementation) of the older arm instruction set. it was originally *intel's* PXA design, and they were in turn were the original "DEC StrongARM" team, bought wholesale from DEC. superscalar ARM architecture was actually therefore first.... from Intel, not ARM! but, marvell's SoCs are now quite dated. none of them have USB3 for example. but, what really kills them off as an option is the pathologically insane nature of the marvell sales team. you'd think they'd want to sell SoCs. can you get them to give you one, even if you offer them cold hard cash? not a chance! this makes it very difficult to actually develop products based around marvell SoCs
in short: the requirements you're after, you're going to need to wait a few more years before aarm64 becomes more mainstream in the server market, and then such systems will be just as expensive, if not more so because they will be in high demand due to their lower power, as x86 and amd equivalents. in the mean-time, consider using something a little less costly, multiplied up in a server farm. save space, save power, save money. there's people setting up fanless servers in a space less than 5in x 4in x 1in that use well below 10 watts... and that's including a SATA-based SSD.
this reverse-engineering has been successful in getting H.264 and MPEG decode up and running. that's a big achievement!
all it needs now is for limadriver to reach its first milestone with even a partially accelerated opengl library and the allwinner A20 will be the world's first truly mass-volume SoC that's FSF-Endorseable.