Proponent wrote:
Is it possible that with the booster engines attached all the way to
orbit, the vehicle could fly a somewhat more efficient trajectory,
therefore boosting the payload a bit? On the other hand, the booster
engines might have to be shut down anyway in order to keep the
acceleration from damaging the structure.

I assume they carefully worked out to the second when the boosters
became a net deficit to the ascent, and jettisoned them at that point.
Although they each generated over twice as much thrust as the core
engine, their Isp was inferior to it.
290 vs 316 in vacuum respectively:
http://www.astronautix.com/engines/lr895.htm
http://www.astronautix.com/engines/lr1055.htm

Pat