blinder,
You were asking about the extended motor tube? Yes, such an extension can reduce the volume that must be pressurized inside the upper stage to achieve ejection. These extended tubes are called 'stuffer' tubes. No, it is probably not necessary on an Omega. (Look at all the big/long rockets like the Richter Wrecker that do not use stuffer tubes and still eject just fine.)
When you get feeling a little more brave, you can start making mods to the plans. You can build the upper stage motor mount to accept 3.75 inch (Estes 'E' motors) and use a one inch spacer for regular D or C-powered flights to hold the shorter motor case in the right spot (cut an inch from the front of a used motor to use for a spacer).
Or plug the front of the motor mount tube and add holes around the sides of the front end of that tube----you have the start of an ejection baffle. Add another baffle plate or two in front of that, and you won't need ejection wadding anymore.
Jim Flis is right (along with the others), you don't tape the motors together on this design. I had about three or four flights on mine before it floated away, and the pop-n-go staging worked fine.