Basically, by adding a wide angle converter to the lens, you will be adding another few optics to the lens itself. When there is another optic there, there is bound to have some light lost... not to mention most of these converter's are not high quality, especially when you get them cheap from ebay. And that would further degrade your image quality... not to mention it will distort your entire photo so much that it is no longer useful.
There really is no need to buy these junks just to get the hand of landscape photography... if you want to shoot wide, use your kit lens' 18mm, it should be wide enough. If you forsee that you need something wider, go to rent a 10-22mm and see if you like it or not. If you like it, save your money and get one.