Is there any advantage/disadvantage to using a heavier spring set and using regulated boost to assist the gate opening? I know theirs an issue if the regulator fails and the gate never opens, but outside of that anything else?
Yes.
With a straight drive pressure controlled gate, the boost will be consistent and smooth, but you have no adjustment without disassembly and the boost curve will be shallower, in some cases very much so. Let me explain...
If you use a straight spring, then the force acting on the valve is simply that of the spring, and as such, in order for the valve to be open enough to control peak boost to your desired level, the valve must crack open well ahead of the point you desire it to be controlled to. In fact, it must crack pretty dang early so that the drive has pushed it far enough from it's seat at full song to bypass your desired boost. The bigger the gate the closer it can get before cracking, but the higher the rate of "bounce" where the boost will overshoot, then undershoot, then overshoot, then undershoot as the gate keeps cracking, seating, cracking, seating over and over...
The only advantage regulating to the top port with an open bottom port has over this scheme is the ability to dial your desired boost level with a simple twist of a regulator knob to artificially change the spring K by applying supplemental pneumatic force in conjunction with that provided by the spring itself. The ramp and onset curve issues remain the same.
The real advantage to boost controlled gating is when you have a multiport solenoid valve that can actually hold the gate slap shut right up to your desired boost and then rapidly start feeding boost from the top port, down to the bottom port to rapidly flip-flop the controlling force from holding the gate down, to slamming it open.
This becomes most evident on a shift when rpm rapidly drops nearly instantly. Either of the top two schemes will have a boost lull off a shift as the gate never fully reseats, and blows a lot of drive out the tailpipe while you're trying to get back on boost fully. In the electronic scheme, right off a shift the controller will slam full manifold pressure to the top port, full sealing the gate so that the charger(s) never lose a beat on a shift, and then shift the boost signal back to the bottom port again as the engine exhaust flow recovers off a shift.
Off a shift it is
very evident if an engine has electronic boost control or not.
On Edit:
If you plotted each on a graph, the spring only, or top port regulated schemes would slowly rise in boost along the rpm scale until peak was reached, whereas the electronic controlled scheme would rise sharply up to desired pressure and run across the graph.
