I’ve always stayed away from grapple & overrun rules, but, this time, i can’t ignore it.
The rules says that
A grappled character who attempts to cast a spell or use a spell-like ability must make a concentration check (DC 10 + grappler’s CMB + spell level, see Concentration)
so does the feat, special abilities, objects granting bonus on the grapple check count against the DC of concentration?
Does the bonus like flanking count?