Any tests showing bigger canisters are better?
I have never tested something like this, but I suspect marty is correct. The stock reservoirs are sized based on the wast gate actuators so unless you move to bigger actuators, the stock reservoir volume should be fine. You'd be most likely to run out of vacuum on a tight road course where there is a lot of on-off throttle driving that spools the turbos repeatedly. I suppose if you were spooling the turbos at low RPM that would be worst case because the flow of the vacuum pump is linked to the speed of the engine, though your RPMs shouldn't be too low if you are driving on a road course.
You could put a tee on one of the lines running from the reservoirs to the solenoids and hook up a vacuum gauge for testing if you were ever curious. I have no idea what would be an acceptable delta in vacuum pressure, but less is probably better. We would need data on the speed of the actuators versus vacuum pressure. Also important would be the clamping force of the actuators and the mechanical advantage of the wast gate flapper arm versus the waste gate area and exhaust manifold back pressure. The actuators need to be able to overcome the force generated by the back pressure on the waste gate plus the internal spring.
Recall that to overcome the spring and seat the wast gate the vacuum pressure applied to the actuator should be 5.9 in Hg (200 mbar below atmospheric according to TIS). Additional pressure is required to hold it closed due to exhaust manifold back pressure.
