In this game Large Lasers are very good when you consider weight, heat, damage, and the laser accuracy bonus. I think I would have kept the Thunderbolts close to a stock loadout with a right arm Large Laser, LRM 15 w/ 2 tons of ammo, SRM 4 w/ 1 ton of ammo, 3 medium lasers, 6 heat sinks, max armor. The mech doesn't operate in a vacuum - it's combining its fire with lance mates so a near stock mixed build with an eye for heat management gives that large laser a good home.