I should have mentioned, added the required plenum will reduce volume available for high pressure air. You'll need to run a reg pressure near the pressure of your last good shot to or lower to maximize the number of shots. Then you'll like want a weaker hammer spring best performance with lower reg pressure.
I have added regulators to two tube based aurgun. I both cases it was to reduce velocity SD not to increase the number of accurate shots.
Have tough about lowering the velocity by reducing the hammer spring more? If that isn't enough the buy a weaker hammer spring. There maybe an simpler solution then adding a regulator.
The spring store has a calculator, you can enter the parameters of the current hammer spring. The calc will give the spring rate for your current spring, for there you can search for spring with the same diameter and length an lower spring rates. The spring store has minimum order, $40 the last time order. The Elf has an anti-hammer bounce design, I am not sure how that design is implemented. It make changing the spring more difficult. Just another possibility.