It seems I'm a wrong on some details even though if those %'s are correct or not. When I said "no penalty", I kinda forgot to state that there's hardly any damage/defense penalty. I based it on someone's comment I've read close to year ago on the subreddit that there's about 1% Damage/Defense penalty per LL difference until you hit the breakpoint (which would hardly have an impact on teams damage). The biggest notable Damage/Defense drops what I've seen happens at 570, 550 and 530LL though.

Sure it's slightly tougher to do the Burn strategy when people are under leveled, but it's still more than doable if the team is doing their buffs and debuffs properly. Any high level Golden Gun user can easily get up to 900k+ damage Golden Gun hit that compensates missing damage others players has due the damage penalties.