[jsz0]

It's very impressive Apple made the effort to do this now instead of waiting for 'off the shelf' parts but the technology behind this is probably nothing to write home about. The biggest limitation of display bandwidth is currently external HDMI/DP standards which are slowly ratified and adopted. For all-in-ones and laptops additional channels can be added and/or clocked higher for additional bandwidth. Apple's TCON is probably a fairly standard part souped up a bit for higher performance to handle the additional channels / clock speeds. I'd be surprised if they are doing anything fancier than this because the jump from 4K to 5K isn't big enough to require it. Close enough that the 'old/current technology' can be stretched. On the video card side as long as the 7870 has enough VRAM to store frames desktop 2D / light 3D compositing isn't that demanding. The trade off here is heavy 3D (modern gaming) at native 5K or even down to 4K is mostly not happening on this machine. Probably a good choice since 2560x1440 at 120Hz+ is a better choice for gaming than higher resolutions at 60Hz.

[H_L]

The jump from 3840x2160 to 5120x2880 is nearly 2x, in pixel count. That is not just a 'tweak existing technology' kind of jump.

TCON's that can handle a single logical 4K display are just hitting the market, and Apple has one shipping today to handle 5K. A timing controller may be simple tech, but it's cool that they're so far ahead of the curve.

[jsz0]

It's a big jump in pixel count but a 2X increase in bandwidth can be brute forced / tweaked out of existing technology similar to DL DVI or DP MST. IMO that doesn't diminish it at all because lots of technical problems have somewhat obvious solutions but actually making it happen is still difficult and expensive. Even more so if you want to avoid making any ugly trade offs in the process.