Scott, as many have posted to end up with ZO it takes the econtrols ECU to be compatible with the ZO system. ZO and PP Stargazer do use GPS as the throttle control strategy, the older PP system used engine speed to control the throttle position.
As for the EFI question separated from ZO, ski boat owners that has tried and posted about an EFI conversion (mostly all trying the Sniper) have given up over not getting it to work. Maybe it's just those that posted and there are several running around but I have not seen a single conversion post that ended in a happy marriage. Given your expertise a carb should be fine and you have a few options, typically a Holley or Weber (which is same as Carter AFB or Edelbrock). Just don't run one too lean, marine engines run richer to stay alive as they are always under load. (and that may be the issue on an EFI conversion, the O2 signal / existing calibration really does not richen the mixture enough, IDK).
As for an older boat, no question they can be improved. Removing weight, relocation key components to move the CG forward will improve the wake along with some hull enhancements to correct the deficiencies of any of the specific hulls (side spray on an older Prostar as an example). Moving CG forward counters an aggressive weight reduction program and in my example project improved the tracking.
A quick read of your thread, you have to decide what is your priority in your hobby, we can easily help you spend money. From what I read you are looking at two paths 1. build a really nice older ski boat that exceeds the original 240 hp performance level and has a functional speed control or 2. spend a fair chunk of change to end up with a ZO speed control boat either new hull or old. For item 1, an SBC can easily be build in the 385-400 hp range that offers plenty of torque and drivability to pull up a skier, boarder or footer. Marine cam or torquey truck cam is what you should settle on.