Rather than adjusting your seat position with a custom seat post, get yourself an adjustable stem. With the ability to quickly (relatively [usually just an allen wrench]) change the angle of your handlebars, you can find a body position that works well for you. As mentioned above, moving your seat back can cause serious lower back pain since the vertical force your legs are applying is too far in front of your core. If your willing to go to the trouble of re-cabling your gears and breaks go ahead and get some high bars! In my opinion high bars are not only comfortable (you sit upright that way reducing neck and core strain), but they are stylish and much safer. With your arms almost straight out (like driving a car) you are much more able to absorb the pressure from a sudden stop, and much less likely to go over the bars in the event of a collision. I honestly don't know why high bars are so rare on new adult bikes. Hope that gives you food for thought.