290 against a full 320 is 2.5 delta so should only lose 2-3% but seeing as many players will be 300-310 anyway It shouldn't be noticeable this is however going off their graph they may have made ninja changes since I'd say give it a go see how it works. The main differences occur below 280 light there is a sharp damage drop off below 280 against a 320 or 40 light diff to anyone else