Hi
Yeah, if you call GeoMipTerrain update in a task it can slow things down. The trick is to call it when you really need to. What I do is call it when my camera moves a certain distance. You probably already have a task to handle camera movement when keys are pressed, just put a distance check on current camera pos and previous pos in the camera movement task(s) and when the distance changed is like 64 units, call the update on the terrain and set your prev camera pos to the current. I also do an update when camera has been idle, just to sneak an update if there was movement but not the full distance traversed. doing the update only when you need to (for visual purposes) really speeds up the frame rate, give it a go !
I’ve used L3DT (free version) to create up to 8 texture layers and using shaders. I’ll write up the procedure for making the textures when I get some time.