OK, so who was I kidding when I said I might leave it the way it was - I went back in and opened up the transfer port through the body on the .30 ST barrel conversion, and the gun is now spitting the JSB Exacts 44.75gr at @86ft/lbs with comfort. No external plenum, and less spring pre-load than OEM. Crazy accurate, too. I don't think I could ask for more out of this one. I had originally planned to have three barrel setups fo rthis action using the Old school FX ST barrels in .22, .25, and .30 - but, I think I may need to purchase another Zelos to go any further, as I don't think I want to change this one (other than mounting the 500cc bottle). I also have one setup with a .25 ST barrel with CF shroud - the barrel tensioning clamp is the only visual difference in the guns They use different valve bodies, and TP sizes through the body(s) and barrel(s)
There is plenty of space inside the shroud to keep the sound level down. No need for anything additional.
I made a barrel tensioning clamp that replaced the front block on the Zelos action. There is a system tying the front cap of the shroud to the barrel end that allows for true tensioning of the barrel. I do not know how much effect it is having, but can say without hesitation that the gun is very, very accurate for me.
I bonded a Carbon Fiber sleeve to the old school (full diameter) FX St .30 barrel to bring it to the 15mm diameter I needed to fit the action.