It sounds like you have carefullly considered any changes you have made.
I kept to the internal dimensions that Wayne specified in my 3 build. I am very happy with the box performance. I added a lot of bracing and I cannot yet hear any colouration or resonances coming from the box. I did not add any mass to the horns. I'm wondering if it's really needed. I was going to use electrical putty which is clay like and not supposed to dry out but I thought it might come loose and end up causing a problem. When you tap the horn it sems to have quite a low resonance, certainly below it's cut off frequency. Has anyone done the damping afterwards and heard any differences?