While yes, accurate does clearly only benefit single shot firing modes, the Storm quality creates a very strange situation here. It is firing "single shot," but the Storm quality turns that into effectively two shots.
I've seen this exact question before, and my response would be to create a new "mode" of fire, where the storm bolter only fires a single shot, and the Accurate trait is represented by a particular ease of aiming down one of the barrels of the storm bolter. In other words, when fired in "accurate" mode, remove the Storm quality. Of course this creates strange ammo tracking concerns.
Still, obviously, a full powered Accurate+storm quality (2x SB shots with +2d10 due to aimed shooting goodness) is simply too effective, even for 1k xp. It provides both a rate of fire > 1, and an impressive amount of dice dropped (which greatly increases the chances of righteous fury). Really, the boosted RF chance here is the greatest threat. 4 more dice dropped is practically (I really don't want to have to recall prob-stat here) a 40% increase in RF chance, which once again is significant (to a point of being too good for 1k xp).
This is something the player should of run by the GM long before even attempting to save up the XP to get it. Personally I would of made efforts to dissuade a player from putting me into this situation.
As a random thought: I don't have the book with me, do those rules allow for scatter to be placed on a storm bolter? Now that would be fun to calculate the total number of hits for...
I think the idea that to use the Accurate trait of this super storm bolter you lose the storm quality on single shot mode is probably the best way to deal with it.
I had toyed around with a RT murder servitor armed with a full auto Flechette Cannon (heavy shotgun with scatter and storm). It was mean and very efficient at it's job of annihilating large swaths of low armored enemy crews. I ended up just making a DoS chart just so I wouldn't have to think so hard with the calculations. I never did turn it loose on the players.
I think the stats I made were: Murder Servitor Flechette Cannon "Meat Grinder"- Range 30, RoF - 5 10, 1d10+6 Pen 0, Clip 500, Storm, Scatter
If I recall it was also armed with a flame thrower, close combat weapon of some kind, and a krak grenade launcher for making doorways. It was designed to deal with my pre-DW version of hordes.