Welding without using filler is called autogenous TIG welding. As you would expect the area that you weld will be slightly thinner than the parent tubes. You see many custom exhausts welded without filler. I guess as long as the reduction in cross section at the welds is stronger than the forces that the pipe will see than your golden. You will need the parts to fit up perfectly. So I wouldnt be without a cut length in your hand just in case you need to add a little filler to keep the gap filled. A good two stroke pipe can look like a piece of art.