With DX9, and five accounts, Ultra across the board was playable at a decent performance.

However with DX11, and the same five accounts, while the game would load and I would consider if playable, it would have been less than fun in congested locations.
Orgrimmar, around the auction house (wasn't terribly populated at the time), was noticeably spikey in performance with FPS jumping all over the place.
A mass battleground combat would not have been smooth; I'd assume the same would be true for a 25-man type raid.

I pretty much only cared about the pvp aspect, towards the end (last couple years) of boxing Wow.
So, as far as settings went, I wanted maximum for view distance of characters and objects.
But didn't mind so much if the settings (eye-candy) was lower for pretty much everything else.
I essentially put the main on Higher settings, and then disabled Shadows, Water effects, Reflections, and turned down things like Spell effects and Weather.
The slaves did the same, but started on Medium settings, and then had view distance type effects cranked upwards to near maximum.

With IS Boxer, setting up a mapped key to run for the current window (and another for all other windows), which runs on any client switch was fairly easy to set up.
And the closer the settings were for between the two, the faster the swap was...
I lowered the mains settings a fair ways, to enable absolutely instant swaps between any two toons.
If I didn't mind a 1.5 seconds of... the game is adjusting effects and I cannot do anything... the main window could have been on much higher settings; I'd have likely gone that route, if PvE instances were still my focus and instant swaps weren't as essential.

Another consideration for me, was the intention to 10-box on the one machine.
I built the machine with that in mind.
So I lowered the settings, quite a bit lower than I'd actually need for smooth 5-box performance, with the intention of not having to change the settings once the extra five accounts were added in.

Warcraft is more CPU dependent than Video dependent, for most of the graphics and effects in the game.
Video ram seems to be the limiting factor, when attempting to launch a boatload of accounts.
I'd imagine both CPU and Video card power, would determine how enjoyable the play experience is... but enough raw video ram is required to even launch the clients.

I was using a 6 core processor, a GTX 670 with 4GB of ram and 32GB of system ram.
GPU-Z showed the video card at 60-70% capacity on DX11 x5 Ultra settings; it also showed the ram at 3.5+ GB, and that in a lower graphic area (Orgrimmar, as opposed to a newer zone with better graphic effects and such).
The cores were not pushed hard at all.
And the system ram wasn't even slightly an issue.
The dual SSDs, in raid 0, weren't being pushed hard.
Here's my system:

Ualaa boxes with:
OS: Windows 7, Professional 64-bit
Case: Antec 1100 Gamer
PSU: Corsair Modular 1050 Watt
MB: Asus Sabertooth, X79.
CPU: i7 3930K (6 Cores + 6 Logical Cores)
OC: @4.5GHz, on Air with a Noctua NH-D14
Memory: 32GB Corsair Vengeance DDR3 1600MHz, CL8, Low-Profile Ram
Video: eVGA GeForce 670 GTX, 4GB Superclocked
OS Drive: OCZ Vertex3, 120GB SSD x2 Striped (Raid 0)
Storage: Seagate Barracuda, 3TB x2

Streaming on Twitch.TV with XSplit Broadcaster
Shaw Broadband 250 (250Mbp/s Download, 15Mb/s Upload)