While I'm not a scratch golfer by any means, I do know a thing or two about mathematics and physics. From the research I've done, a regular flex shaft is definitely the right shaft for your swing speed of ~90 mph. While many other components will come into play (such as launch angle, backspin and side spin) a fairly good estimator for your driving distance can be achieved with the following formula: Take your swing speed, multiply it by 2 1/2, then add 10% for roll. Thus we have:
Driving distance ~= 1.1(2.5*SS) = 2.75*SS
With a SS of 90 mph, this would yield 2.75*90 = 247.5 yards. If we compensate for just slight side spin or just barely too high of a launch angle, 243 yards sounds just about right. However, distance is absolutely a function of swing speed. You might be able to stretch a few more yards out of a perfect swing, but I think you are doing quite well. One thing I would check is if you can find a launch monitor, make sure you have the right loft on your driver. You want to be launching the ball around 15 degrees, any higher than that and your losing yardage with your SS, any less than 13.5 or so, same thing. Depending on your attack angle, I'd estimate you probably should be hitting an 11.5 degree loft with regular flex.
Hope that helps