In Evanix that gauge port has small like 1.5-2mm orifice that turns back towards the front of the gun. It would be a very poor air route and size for plenum extension. It could be different in Krals though.
I think you should state your power goal. At 12 fpe most anything will work as the existing plenum is enough. At 100 fpe, well let's just say I have damaged a depinger with only one 4.5mm hole in it.
Think of it as pressure rather than air flow, The smaller reservoir is supplementing the pressure in the main reservoir. Everything else works the same as before.
Generally, you want the air inlet from the plenum to be larger than the throat of the valve, so that the air can get into the valve faster than it can get out.... I had two 90 deg. turns in a .25 cal Disco pumper, with a valve arranged like this....The 16 gr. CO2 cartridge had an internal volume of about 20 cc, added to the 7 cc in the valve and gauge port, for a total of 27cc.... However, it fed into the very front of the valve, just behind the check valve, and a long ways from the poppet.... the valve seat is about in the middle of the rear valve half.... That gun could reach 40 FPE at 1500 psi, and shoot efficiently at 30 FPE at that pressure, with a 20" barrel.... The passages from the CO2 through the gauge port and into the valve were 1/4" however, which is bore-size for a .25 cal.... Bob