I agree, ML, a most interesting situation. Year-round Tilapia we don't have much direct experience with here - possibly none except for the Thailand Twins, and Rad and Don sure don't have any LMB.
Your WAG for small LMB and large Tilapia removal sounds good enough to be a SWAG to me. Think introducing adult BG might re-establish a 12 month forage base, maybe after enough of the above culling?
Winston/ML: How big is the mouth on a 1 to 2 lbs Tilapia? (What could they eat if primarily piscivorous?)