Yeah it sounds like a nice solution. I also like that it affects co-op, since it somewhat limits the possibility of an OP summon just nuking the boss with the host barely doing anything. Although that's inevitable to some degree, since min-maxers will still be notably stronger than a lot of hosts.
As I understand it, the instant you have a +5 of whatever in your inventory, you are locked to +5 for that playthrough as far as online play. There's no way to revert back without starting a new game.
So if you upgrade your weapons you do risk locking yourself out of a lot of early game PvP/co-op even if you stay at a low SL.
SL 70 sounds awfully high for just the second boss. I would doubt a lot of people are in that range at that point.