David Flynn

If you do away with the backstay you'll have to do something else to hold-up the mast. Typically this means a pair of top mast backstays which need to be manipulated (one pulled on while the other is let off) every time you tack or jibe. On cruising catamarans the upper shrouds are led way aft to support the loads which works, but also restricts your ability to ease the mainsail out. You should also remember that adding sail area to the back-end of the sail plan will add to weather helm. You may be creating a balance problem by adding as much area as this could permit.   

Also, if you're going to race, you will take a hit under PHRF rules for excessive upper and mid girth on the main. You should look at the aspect ratio of the existing rig. We build mainsails all the time with enough roach to overlap the backstay by an amount you can live with and within PHRF roach limitations. You could achieve your goals without a major re-rig. Another less extreme approach would be to add a crane at the top of the mast that moves the backstay out significantly and makes more room for roach within PHRF girth limitations.