You're correct in assuming that the chambers hel decrease weight... obviously. However the one thing in your post I want to draw attention to is the fact that you said it would help sustain... In a way yes... but essentially no. The chambering of the body will increase resonance. The difference between sustain and resonance is sustain is the lenght of the specific note... and resonance is the loudness of the note... People can mistake resonance for sustain because a semi hollow body will make the notes louder, allowing you to hear the note longer... but in reality it is still vibrating at the same frequency... you're just hearing a range of frequencies that a solid body guitar wouldn't normally resonate..... It's like comparing the "Sustain" of an acoustic guitar to the "sustain" of a solid body electric... the solid body electric doedn't allow you to hear the frequencies like the acoustic... but the string is vibrating in the same fashion.... Obviously string gauges are different and scales are generally slightly different.... But if you were to do an expirement and make all variables such as scale, string gauge, and intonation constant... you would find that the acoustic still appears to have more "sustain".... if you measured the vibrational frequencies however you would find that both string would vibrate at the same frequencies... But the solid body acoustic is unable to resonate certain frequencies.....
There are also differences between a Semi Hollow body and a chambered body.... A semi hollow will have much much more open space within the body... and generally takes more of an acoustic sound. With a chambered body though, the chambers are designed to keep the over all tone of the guitar but enhance the qualites of the tone... That's why the chambers are broken up and not one solid chamber. This keeps the over all tone of the wood and the guitar... while enhancing the resonance of the guitar and reducing the weight... Honestly... I would never buy a chambered Strat body...... I would however spring for a chambered Les Paul... The reduction of the weight would be very much welcome. As for the tone... generally chambered bodies get a little bit warmer of a tone than a solid body guitar does... But it also depends on the specific woods...
