Thanks, yes very odd. I've swapped intercoolers and checked all the vacuum lines with no obvious leaks but will be replacing all vac lines this weekend. No obviously loose wires on the ECU harness. Nothing else was changed on the car whatsoever.
Yes, totally appreciate I was tempting fate by testing the chipped 6K. It was maxing out at 15psi when ~5 degrees out, otherwise ran about 14 psi. I suppose it's possible my MAP sensor was damaged and the replacement was also a dud.
I put the 6K back in today as the 3B was causing the car to be undrivable with what felt like intermittent signal from the starter switch; it seems to run better, though still won't boost smoothly.
I'll check/replace lines and ensure plugs fitted correctly over the weekend.