Apple doesn't let you replace the default keyboard on iOS devices. That is a big bummer. While the keyboard's auto correction has improved in the last few iterations of the OS, there are still ways to improve the accuracy and typing speed of these virtual keyboards. The issue with stock iOS virtual keyboard is that there is no feedback from the keys. You are just hitting different places of the screen. But I'm sure, with time you can improve on your typing even with the default keyboard - as I have.
When I bought my first touch screen device, the Samsung Omnia i900 running Windows Mobile 6.0, I had trouble typing quickly. Well, I've never been good with the keyboards on my old phones either. Maybe that was because I didn't have a girlfriend to frequently send SMS to. With the Omnia, there was haptic feedback to give you some kind of feedback when typing. But it wasn't good. I felt the keyboard was laggy with it enabled so I always disabled it.
That was when I watched a video on Swype, a new keyboard that worked by swiping on top of the keys, instead of hitting them. You didn't have to swipe perfectly on top of the letters nor had to be very accurate with the order of the letters. It was truly "intelligent". You could insert new words by going them in. Luckily, there was a compatible port for the Omnia. Not an official one though. I installed it and was impressed with it. Very impressed!!!
But I got rid of the Omnia before I came to Japan and switched the iPhone. IPhone 3GS was my first iPhone. Then the iPhone 4S and now the iPhone 5S. So I had been using the stock iPhone keyboard since 2010. But if you could Jailbreak your iPhone, there is a version of Swype (but it's just a mockup and not very functional) on Cydia store.
