From what i have read on the first datamines, mercury gear will be 300-310. All other below that (guess 290-300).
TBH, i don't want mercury to give 320. It's fine that (hard) raid is the one that gives highest light level gear. Just don't make it so damn rare. Esp since you need good ''numbers'' to roll, plus good perks plus multiple stuff for infusion of other gear.
Take gloves for example: you kinda need multiple for different primaries, otherwise you are not perfected. Same for bonus ammo on chest/boots.
At this rate, i won't even have a full set by december, lol.