Given the ongoing fire sale of these devices, it seeming increasingly likely that they'll be well supported, with more than a few people having bought extras to tinker with. I figured I'd get the ball rolling on the topic of adding external antennas for improved range over the internal ones.
I'm still waiting for my own devices to arrive, and will be traveling for a good while after they do, but here are some questions to further the discussion:
Which connectors are for which radio? This pic shows 6: the 2 on the far left and the 4 right of the middle.
What would be the best place for drilling and mounting external antennas? Originally I thought mounting toward the top sides of the white case, but it seems to me that with the case's apparent sleeve-like design, that would lead to having to disconnect all internal antenna wires before completely removing the board from the sleeve, and maybe it would be better to mount them on the black plastic frame at the top -- though might mean ditching the white plastic grille that goes on top.
Sources for good external antennas compatible with 2.4 GHz and 5 GHz Wi-Fi? Price being somewhat key, since I'm not in a rush (AliExpress is fine) I don't want to spend more on the antennas than the actual device.
802.11ac and newer relies quite a lot on Mu-MIMO and beamforming, this depends on proper antenna orientation to be effective. Apart from the regulatory concerns (modifications void the certification and make operating the device illegal in most jurisdictions), it's very hard (~impossible) to improve the situation, without lots of trying (different feed lines, antennas, orientation) and measuring the results with rather high-priced equipment. It's much easier/ much more likely to make the situation worse, as finding good/ affordable components is rather hard and requires testing lots of alternatives (unmarked offers from random sellers abroad without data sheets are very questionable).
Not to the same extent, yes 802.11n already had antenna diversity, but that's much less complex than Mu-MIMO and beamforming; the quaility concerns of 'cheap' feedlines and antennas especially for 5 GHz) does apply though (it's very hard as a mere mortal without measuiring equipment and choosing from a selection of a few dozen suppliers which of those meet requirements or make the situation worse - and the placebo effect doesn't make this any easier).
