Wayne,
I put a dual-port wastegate actuator on the CSX earlier this year to control the internal wastegate on my GT3082, but with the transmission issues at SDAC and after (re-making it a 520/555 hybrid and replacing all the bearings/seals) I haven't had time to really test it out, so the "other" port is not hooked up yet, hence I don't have any real data for you.
The idea is to get a lightly-springed actuator to get a lower minimum attainable boost level, then use the second port to hold the wastegate shut to attain higher boost levels than the spring alone could hold.
I went with the Turbosmart TS-0620-1143 IWG75 wastegate actuator because aside from having dual ports, it has replaceable springs so you can fine-tune the minimum boost level:
I plan on using it with my three-stage grainger-based boost controller, with the first two stages connected to the main port for low and medium boost (within the range of the spring), and the third stage solely used to hold the wastegate shut by providing pressure (through a regulator) to the auxiliary port.
This is the configuration that Reeves used to raise his boost level on his external wastegate last fall, and it worked well for him, so I want to duplicate it with an internal wastegate.
I hope this helps meow,
Warren
P.S. If you do get this actuator, the threads on it are M6 x 1.0, so make sure you get a clevis with the correct threads.