Headstay sag is a function of its length, diameter of the wire and tention. Shortening the headstay an inch or two won't much affect head stay tention but, as Bill points out, move the mast forward - which will move the CoE to help balance. On the J/30, backstay tention will only marginally help headstay sag as most of the force of tentioning goes toward bending the mast. Some have even argued that bending the mast causes the center portion of the mast to move forward, thus inducing more headstay sag. My conclusion is that I don't get any noticable improvement in headstay sag from increased backstay tention.
As for depowering the main, backstay tention will flatten the top 1/3 of the sail and open up the leach. Enough backstay tention will actually invert the top of the main. The right compbination of traveller adjustment and backstay tention will go a long way toward balancing the helm. A de-powered main and a balanced helm are a lot faster than an overpowered J/30 and make it a lot more comfortable to drive.
And as an FYI, while I can't speak to current Dacron sail design, the original Dacron mains came with a flattening reef about 6-8" up the leach. The old rule of thumb was for 16k of wind and above - strap in the flattening reef. The flattening reef pretty much took care of the bottom of the sail - the backstay took care of the top. A blade for a main, traveller down, full crew on the rail - the old mantra was "which number 1 are we putting up today?" (as in; the old blown out #1 or the new - let's save it for important races #1)
