loads look like 87.5 gr r15 that does not fill the case so a filler needs to be used. Can one substitute r25 or retumbo to fill more of the case?

no reason why you can't, but be aware that a change in powder may affect the regulation. H4831 is another powder that works well. thats some of the fun of reloading - seeing what does or doesn't work well

Butch Searcy has an IMR4831 load for his rifles as well. If you're shooting a Searcy, contact him.

The slower powders have perceptably greater recoil ... honest!