I've drilled the threads out of the transfer port. I was going to put a threaded and drilled piece of aluminum bar in the but I feel it's just extra work to restrict it again. I've also drilled the barrel port and polished the hammer.
I've seen someone that polished the contacting surfaces of the washers in the regulator and was getting an average shot spread of just 1 to 2 fps! I'm going to do this soon.
I am the maker of those plenums (alienairgunsupplies) and the listing has been updated with an accurate, calculated volume of 15.75cc. I've had one customer fit one to a stock .25 and he got a 6fps boost without doing a thing. Others reporting being able to drop the regulator pressure by 400psi and still keep the same pellet speed that they're happy with.
I can make them bigger, bit it's about how much you're happy with sticking out the bottom of you're rifle. I'm about to make one that's about 27cc for a customer that already purchased a 15cc one.
I have bought a stock avenger because I don't really have before and after results for myself. I just have to get the time to shoot some groups over the chrony.
Let me know if I can help you out at all.