when the car is cool, get a good strong set of pliers and grip the actuator rod and try to pull it towards the dump pipe. Is it firm to really hard to move? does it wobble at all or does it feel loose?
if it all feels OK
to bring the boost on sooner try using a turbotech boost controller as the ball and spring mechanism will not net the actuator open at all until the desired boost is reached. it is an easy way to eliminate wastegate creep.
your power figure is really good. but you are right there seems to be a fair bit of lag. you have given a lot of useful info unlike a lot of the "what is wrong with my car" threads.
Also check to see if all the clamps leading to the actuator are tight and there are no splits or pinholes in the hose.
boost control looks good from the graph what are you using to control it?
Have you still got your RB20 actuator? it will suply more preload and may also help spool the turbo earlier.
worst case scenerio, you could pull the dump pipe and check the flap is seating properly.
Hope this helps