Super_Stanchy can make the RVA cheaper than I can
There is a long list of things that can be done. Some as simple as tuning the valve for better efficiency. Others are more complex, such as slots in the feed tube to see the remaining paint. Others are far more complex, such as spring feeds and barrel replacements.
I highly recommend you poke around in the Sheridan section of the Armory and look at pictures of what people have done. About the only limit to what you can imagine into a paintball marker, is *usually* your wallet