Stockclass: It can be done, there is a "prototype" in the works at the moment.
12-gram: If you want a brass valve tube, then Rick can install a dropout slot with a PPS fasst changer. Stainless..not sure at the moment, Rick doesn't do lever dropouts anymore. If you want a stainless valve tube, a vertical air adapter with a CCI bucket changer is an option. Rick just built a Viper with a VAA, it came out really clean looking.
Efficiency: the original vipers tuned to run off 12-grams were getting 40-50 shots after some of Rick's magic.
Servicing: Same as any standard sheridan. They use the same valve with some valve work done by Rick.
Cost: $500+ depending on options.
I will bring my Vipers to the next pump game if you want to take a look?