It seems that everyone is talking about learning FTG.... My first thought was that it is the next stage in being super fast.... Lets say Minato was already fast as the raikage, but then just learned to go beyond that speed..
or... another thought: if you were to think of all of this as real world physics: the faster an object moves, the slower time is perceived... until it goes faster than the speed of light, in which case time is reversed.. of course this is in simplest terms... i think this could explain a thing or two.. it might explain how minato was able to travel so quickly. If he were traveling at the speed of light, an observer would perceive it as instantaneous and believe its teleportation. Now, it is called the Flying Thunder God Technique, making me think that there is some lightning release involved in it; and, that perhaps he was able to use lightning/electricity to enhance his already great speed.. The use of seals could just be that minato was unable to control where he would land on his own and that just helped him in that regards (im sure its quite difficult to land in an exact location when traveling at that speed).