I have no experience with a Benjamin Marauder, but I bought an Air Venturi Avenger in .25 and use JSB Diabolo extra heavy 33.95 gr pellets. I pump it to 4500 psi and have the regulator set at 2800. @50 yds I made a two 5 shot groups that each had all the shots touching. Impressed the hell out of me.
The only thing I dont really like is I use a manual pump and it is a chore to fill it. The shots stay very consistent until the pressure drops below 3000 psi which is about 15 shots.