Nice stand!
As long as you have a "standard" prop -you can try different mods to the engine and you will then see what power % increases are occuring
changing props proves ZERO about the engine - unless you know what the hp is .
for example :
on testing electric motors -we look at thrust (as you do) but also the watts required for that reading .
So if I change props and see more thrust -I will always see more watts being consumed.
Irrespective of the prop used
If you change props and see more thrust -at the same rpm - you will obviously be getting more power from the engine by simply changing the prop.
How can that be?
Prop efficiency at zero airspeed is the only answer .
Where the thrust stand will really be of value -is in comparing the engine mods -with standard loads you use (props you have that have a known static thrust at a certain rpm)
Or - simply use it to tune a given setup -
different props which provide different thrust at different rpms will tend to confuse which does what except if you do all your flying in a fixed hover.
anyhow - nice setup -it can provide lots of useful info.