Yes Not having a full johnson bar is known as cutoff

This will work a bit on Gauge 1. Not having the reverser all the way forward can increase efficiency and give
a bit longer run times, particularly with large boiler locos

 

It seems to help save steam on my aster mike. Was playing with it the other day to see how low I could go. Got to the point where I was running still but draft was dropped so low I had to open the blower kinda killing the whole point.

 

does that same situation occur with a Gauge 1 locomotive and would it be a useful way of controlling the locomotive instead of all with the throttle/regulator?

Gauge-1 is the common designation for a 1/32nd scale model of a standard gauge loco. These models tend to be very well engineered, expensive, and yes, some of them will allow a modicum of 'cutoff' using the Johnson Bar.
Other scale locomotives, for example the 1/20.3 models of 3' gauge locos supplied by Accucraft, or the 1/19th SM32 scale models of 2' gauge locos from Roundhouse, have "simplified" valve gear, and the manufacturers usually say that they can not run using the reverser for anything other than full forward or full reverse.

 

The Accucraft 'Royal Hudson' has a Johnson Bar with a reduced setting from full. I've often used it when I have a full consist and it still gives a great performance.

 

It depends on the valve gear. Something with prototypical valve gear should have the same attributes as the full scale version.
My Ruby from Accucraft does not have reversing valve gear, but a piston that will reverse the supply and exhaust ports on the cylinders. This valve paired with a servo will act like a combination reverser/throttle. But then it gets jumpy. And that style of reverser doesn't provide varied cutoff timing like actual valve gear does. These style of engines see more efficient operation by putting the J-bar in full forward or reverse then only using as much throttle as needed.

I can see the argument to be made that even with full prototypical valve gear, you need a combination of varied cutoff and reduced throttle to find optimal efficiency.