In principle, you can answer this question by finding the in-game location coordinates (treasure trail coordinates) of the northernmost, southernmost, easternmost and westernmost points on the map. A quick look at the locator learns that the present continent spans from 30° North through 20° south. That means the continent takes 50/180 of the planet on the north-south axis. On the east-west axis, the present map spans approximately 55°, thus 55/180 of the planet. Multiplying, we see the present world map takes 8% of the entire runescape planet. I don't think so. They may make it so that you end up in the east when you go far enough to the west and vice versa, but since you can't cover a spere with square tiles, I don't think they'll make an actual sphere.