It's like you say Jack. It happens that the valves operated by air pressure, spend air for their operation, regardless of the air used to propel the projectile. If the valve is opened by a mechanical, magnetic system, or something else, the air consumption will be only that necessary to drive the projectile, that is, less.
This "premise" works isolated from other contexts such as ease of construction, costs etc.
An example that has always seemed educational to me, is the case of the famous glow engine of aeromodelling 0, 49, which is a small two-stroke engine, without carburetor, (in the full sense), without rings, without bolt on the piston. .. a great simplification, which makes it possible to fly some things with an engine that is almost a "functional diagram"
To my mind, fans have opted for a solution that allows almost anyone to build a piston valve. In reality as a whole, where several factors appear apart from just performance, it is an excellent solution.
This "simplified" engine has a few constitutive parts, compared to a motorcycle engine, which has many more and yields more.
Performance itself is not everything. I prefer to evaluate these things based on a more general context, where aspects such as ease of construction are as important as thermodynamic performance or compressed air consumption.