A good reason to play group play if you want to plvl in the next patch, calculation included. Taken from forum :

Lets say you're in MP10 and have a base of 500% bonus experience

Addative (old method):
500% base + 1 stack of neph (15%) = new bonus exp of 515%
500% base + 2 stacks of neph (15% + 15%) = new bonus exp of 530%
....
500% base + 5 stack of neph (75% total) = new bonus exp of 575%

Multiplicative (new method):
500% base + 1 stack of neph (* 1.15) = new bonus exp of 575%
500% base + 2 stack of neph (* 1.3) = new bonus exp of 650%
....
500% base + 5 stack of neph (* 1.75) = new bonus exp of 875%

add on another 30% for 4 player bonus:
500% base + 5 stack neph (1.75 + .3 coop bonus 2.05) = 1025%

At 4 players co-op we're looking at a whopping 1025% exp. Who needs scorps anymore.