couple of questions.
1. Are the pods any good?
Not bad, and a useful improvement on the unassisted hub, though unlikely to be as good as a dedicated mesh wifi system that's been purpose designed.
2. Would a pod in the same room as my laptop generally be better than just using my router on a different floor (as currently)?
Possibly, because a big impact on wifi performance is distance from router to the relevant access point. However, the pods use wireless backhaul links to the hub, and therefore act as a sort of relay, and performance may be better positioned between the hub and the device - experiment and find out.
3. I understand that the pods require the Hub to be in router mode. Has anyone managed to get them to work with it in modem mode in conjunction with a third party router?
No, they won't. They only work with a VM hub that is running the Plume emulation software. And therein is the problem, for the pods to work you need to give up all control of your wifi to Virgin Media, and rely on the very low spec hub to act as router. For casual users (ie the majority of people) that's fine, but you can't expect them to give you the capability of a top notch router like your AC86U. Personally I'd buy a second compatible Asus router, and configure that as part of Asus' AiMesh system, rather than rely on VM and their technology.