I'm getting a slight hesitation at around 4500rpm in 5th and boost is around 15psi. This happens on WOT. Doesn't seem to do it in 6th (well, not that I notice).

Could this be mild fuel cut or a coilpack, do you think?

Standard BB turbo.
Actuator replaced by an adjustable one.
As far as I can see, the wastegate is opening/closing fine.
Boost controller is the SBC i-D3. I'm fairly confident its not the controller, as I've done fairly extensive testing with it off to try to resolve the boost surging I've been having.
Does it with 97 and 99 RON petrol.

I'm suspecting it may be leading to the surging.