I've got a 2860 as my main router. As has been said, it's pretty advanced for a home router with decent vlan, support for site to site VPN, qos etc.. However, I found that its modem doesn't play that well with broadcom cabinets, particularly if you have vectoring enabled. So, my first step would be to move to a 2 box solution. Pick up a VMG8924-B10A on ebay for £10-£15, put it in bridge mode and use it as a modem on WAN2 on the 2860. Connect a second LAN port back from the VMG8924-B10A to the 2860, and you can monitor the stats etc for the VMG8924-B10A, which are far more extensive than what you get from the 2860, you can use dslstats etc..
I've recently made that change, and it's a significant improvement on the 2860 built in modem. Then, you've got a really good pairing of a good modem and a good router. If you're still having trouble, then you're just looking at the wifi situation.
I have a couple of AP-902s hooked up to my 2860, along with a couple of Archer C7's running DD-WRT in AP only mode. I chose the 902's originally because of the simplicity of doing vlan tagging on wifi ssid's - through a UI, rather than startup scripts on the C7s. However, I don't have any problems with buffering or access on any devices. One thing I do know about Android devices (I don't have anything apple) is that they're really bad at dropping one access point and connecting to another when the signal strength is low - they hold on way to a weak AP rather than switch to a strong one on the same SSID. However, if you've tried it with only 1 AP, it's likely not that.
Do you have different SSIDs to 5Ghz vs. 2.4Ghz. Do you know if the laptop and the mobiles are connecting to the same SSID and/or frequency. Do you get the same buffering problems on both 2.4 and 5ghz?
I would say, give the VMG8924-B10A option a try anyway - it definately improves the 2860 but likely won't fix the wifi problems on its own - but it'll only cost you a few quid and you can always sell it again later. Then you've got 2 decent devices doing their own thing, and you're just down to fixing the AP problems if they still persist.