Adding a HUMA regulator will only help with you spread making sure your shot is consistent. That's what a regulator does, it regulates the amount of air flow. It may not add shots ( but can). Many have not had to add this feature if the shot spread is consistant, (but I did). At 880 fps you will only get around 16 shots. Doing the mods I suggested and adding a regulator, I get 28 consistant shots averaging 844 fps. I have followed other's example and have not tried to get more speed (therefore power), I wanted more shots. At 840 fps, the .25 Marauder shoots a 25 gr. pellet at about 40 FPE. This is plenty of power for small game out to 75-85 yards. I can stretch it to 100 but the wind can be a factor. Trying to get 880 fps (therefore power) would result in reduced shots. If you are looking for power meaning, 45 FPE and above, I am not sure I would do all this mods right away. AJshoots was able to tune his .25 Marauder with an extreme spread under 5 fps at 50 FPE but only got 8 shots. He did enlarge the transfer port. This was his coyote rifle, he didn't need more shots. You will have to decide what you want it to do and tune it accordingly. I am happy to get 28 shots at 40 FPE.
The good new is that this rifle is able to accomodate a lot of needs. Once you understand your purpose, that will help you determine the adjustments you need. Travis at WAR can be a great resource. The upgrades he offers are very affordable and you can do them yourself.
Best of luck with your new project
Doc