I have a Balsa SB that is built and ready for primer. I had been thinking about keeping the weight to a minimum too. I took some accurate weight measurements to see what can be done:

Speed brake: 4 oz
med servos 1.4 oz (for elev, ail, flps)
full size servo: 2.1 oz (for elev, ail, flps)
BVM SS pipe weighs in at 12.8 oz, Tam MAY be able to make a smaller one for a PST600R around 7-8 oz.
BVM bypass weighs in at 7.3 oz, may be one can delete it for an open bypass.
Cockpit and clear canopy is 3.1 oz w/o pilot.
Use 2 1/4" wheels for 2.5 oz less weight.
Fill main tanks to half only, the 600 will fly 7 min on 55-60 oz (I am told by PST).

I would personally leave all the doors in.

I have added all of the above weight reduction including the lesser fuel capacity and I come up with about 3.5 lbs minimum (this is all other things been equal such as paint, servo ext..etc)

This will yield a SB at about $19-20 lbs wet (my last composite bandit was 25 lbs wet with 24oz of smoke). I think that would make a super super bandit!!! I may try it myself....