Personally I would always upgrade the level of scope before the magnification or tube size... probably even the objective size... especially if your are shooting a older leupold or like a VX1. Meaning if you have a VX1 go to a VX2... the quality of glass makes probably makes the most difference.
I just mounted a Leupold VX2 3-9x50, 1" on a gun yesterday. Sighted it in today. IMO that is a great bang for the buck scope (around $475). I have several of them on guns. It is a definite upgrade over the VX1 and good enough to not spend the money on the VX3 for most of my guns. Its a great low light scope IMO.
Every thing you ready says a 30mm tube will not help in low light.
In my experience any thing over about 8X is also a waste in low light. I use my 14X for sighting in, target shooting, and the occasional coyote in the middle of the day. Most all my shots at animals are low light at hogs and deer on around 8x. On my 14x Leupold scope the dial pointing straight up is 8x. Its easy to set even in the pitch black dark.