So, somewhere in the forums,I read that a game was developed which used the coords of the player in the border and spawns it in the relative position in the next level. Is the level loading time small enough to look seamless?
No not in 5.3 or 5.4. Depending on the amount of meshes, particles, audio and materials etc... used in the level it will take some time to load up the new one. The less content you have in, the faster it will load.