I'd be very reluctant to increase the size of the headsail on a 260. To expand on Benny's post, a 260 is driven more by the mainsail than by the headsail. I find my 260 sails well on main alone (just the right amount of weather helm). It's a delight to sail with both sails up in winds up to 12-15kts with full sails. I'd think that if you increased the size of the headsail, you might have difficulties with lee helm (which is BAD...especially if you solo) and keeping the sail plan balanced. I'd go with the A-sail over a larger headsail.
You didn't say what year your 260 is, so you might be better off seeing if your main still has a good shape. It might be time for a new main. Also, make sure you've read up on properly trimming your sail for light winds. I've found that a little trimming goes a long way. Not only is it the angle of attack (angle of the boom to the wind), but the camber of the sail. If you have your sail too flat (outhaul pulled tight), the sail won't perform as well in light air. If you ease the outhaul a little, it'll fill the sail out a little more (more camber, or belly in the sail) and should power you along a little better. So, you might be better off, if you don't have it already, running your outhaul back to the cockpit to make it easier to trim the sail (same with the topping lift). So, before you spend any money, try being more attentive to sail trim (Ok, you might know all of this, but heh...this is what forums are for...lol).
Final observation...when you're sailing monohull clorex bottles like a H260, trying to be a speed demon is like putting a spoiler on a Volvo 240..