Switch to a stiff shaft?

Hi, I have a very fast golf swing and have been hitting my cleveland Hibore driver about 300 on average and its a flex shaft (when i miss it, its a pull), I just bought a stiff shaft cleveland hibore xls on ebay and was wondering if it would make me hit it any further? I played on sunday and hit my drive 320 right down the middle on a hole but i cant imagine hitting the ball like 340 , i dont know...... any ideas?