I'd put pressure cooking before sous vide when it comes to lamb neck if the end result is going to be flaked for a dish or needs to be super tender.
Thinking about it more you're probably lying right about the time for the lamb being on the low side. However, I wasn't aiming for completely fall-apart meat and as it was cubed rather than a whole piece I don't think 16 hours is realistically going to be required. Maybe 8 though possibly less with a higher temp.
Whole or cubed won't matter a jot - with the temps and timings we're dealing with there's plenty of opportunity for a whole fillet to do its thing. I just think that sous vide is the wrong method for the result we're aiming for here.
I spent far too long trying to replicate a Jason Atherton recipe which involved sous vide lamb neck (78° for 8-hours) and eventually switched to braising it on the advice of his sous chef, who I happened to meet at a thing one weekend. The lamb neck on this particular dish was supposed to be so tender you could cut it with a spoon and it would just melt in your mouth, but have enough body to it that it stayed as one whole piece.
If memory serves me correctly, Alex mentioned they cooked the lamb for 18-hours at 76° and it was only through doing this that I achieved the texture I was looking for and that I'd had when I eat the original dish.
But having experimented to the point where I didn't want to eat lamb neck again for a
very long time, I found that braising for 3-hours at 135° gave me the exact texture and flavour I'd been craving.
To be honest, most of the time I'll stock up on lamb neck fillets when they are reduced and use the pressure cooker. An hour or so in there and they are meltingly tender, but you don't get the same infusion of flavour that you will from a braise. Easy enough to sort that post-cooking though.