I think it may have something to with the tension the wastegate actuator imposes on the wastegate.
To little tension and the wastegate rattles around like you mentioned and this also means the wastegate is partially open which explains the delay in boost.
It really makes no sense how it happened when you swaped actuators which are phycially the same dimmensions though.
I had a similar problem when I swapped turbo's. The core of the new turbo was shorter than old one, which made the actuator rod too long. My solution was to weld in a cable tensioner, which works in the same way as the HKS adjustable actuators.