This is one of the external power adjusters I make and use
And I will tell you without a doubt with the stock spring I get the FPS down to 0 and up to 968.2 with the stock spring.
There is no need to change out the stock spring.
I do make internal adjusters for a local club that teaches a safety shooting class to the youth, they require a set fps as there young inside shooters.
In order to change the FPS on the internal adjusters there is a need to change out the spring but that is by design. The only reason the internal power adjusters are adjustable is to maximize the installed spring.not to set the FPS.
I adopted the BMW spring color scheme as I do love my BMW and found the study of spring characteristics quite fascinating, yellow, blue & green for the BMW.
I could say yes you are correct on the internal powers adjusters a spring change is needed but only because it is design that way.
I could make and sell the internal powers adjusters for the open market then offer a whole line of springs to set the FPS at $5.00 bucks a pop but what a waist of hard earned cash.
It's better to have an external adjuster that utilizes one spring for many different settings.
With a true understanding of how springs work you can have it both ways and not as you would advise.