i was looking somewhere online a while ago and forgot to favorite it. But it was a wastegate that had preload on it so you could "control" lag. You set the preload higher and the wastegate wouldnt start opening till right @ your desired boost leve.... reduce the preload and it would start opening sooner for a more linear boost curve.

 

AGP makes one.. basically any wastegate can be setup to do preload with a threaded actuator arm.
It's not as linear as you might think, but it will do the job..

 

hum like this one??

 

yup i think thats something like it. The one i remember seeing was a 38mm silver W.G. but assume that does the same thing.