I'm afraid that medical evidence pretty much agrees that bracing cannot permanently correct a spinal curve once growth is complete. In fact, there is little evidence to support the idea that bracing can permanently correct a curve in an immature spine either. Most braces aim to simply hold an existing curve and prevent it from getting worse, and even this usually requires wearing the brace for much longer than 8 hours a day (16-23 hours is the norm).
The good news is that you are young and fit and it sounds as though you enjoy exercise
there are certain types of physiotherapies that are reported to help with the cosmetic appearance of spinal curves - you might like to learn more about these. Schroth therapy is the best-known of these - you can learn about the exercises on the web, or find a registered Schroth therapist using the stickied post at the top of the nonsurgical forum. There are also special Yoga for Scoliosis instruction DVDs available (look for the name Elise Browning-Miller), and The Alexander Technique can be very beneficial for postural problems.
If your curve is truly postural kyphosis then exercises could potentially help a great deal, as the curve should resolve completely if you consciously hold your spine straight. If your curve is structural then things will be more difficult. The most common type of kyphosis, Scheuermann's Kyphosis, is caused by the vertebrae being wedge-shaped and exercise has limited potential for correcting this because there is little you can do to change the shape of the bones.
If you do decide to try a brace, remember to keep exercising as much as possible because bracing can cause the muscles to weaken if you aren't careful.
Good luck with whatever you decide to do! We will support you whatever your choice.