shouldnt have to add lead to the plane to get it to balence. move the radio gear around to obtain proper CG. The object is to keep it light as possible. no dead weight. . did your center wing spar have 3 pieces of ply? mine has 1 that looks 1/4 in thick.
The carbon tube wouldnt do much beefing tin the servo wire hole unless you stripped the covering and center sheeting and braced the ribs there. The wing is built very strong. Glass clothed center would be about the best bet here. and the good thing is when your dont the belly pan covers all of it