I installed my Kelleners and SS x-pipe myself with some friends about three weeks ago and we just used the supplied clamps, and I dont have any issues with leaking, I believe if you weld it, that you may have to drop the whole exhaust just to properly weld it, and doing so may make it harder to remove in the future for other Jobs like clutch rplacement and what not, the clamps should be fine, Josh
Sorry to have to disagree with the majority here but welding will always be the better way to go. If you think you can just unclamp an exhaust and change it out, guess again. If the clamps are tightened properly in the first place to prevent leaks they are going to crip the pipes, making them very difficult to get off easily.
Cutting the pipes is much easier and welding will provide a longer term solution to leaks than clamps. I am sure there are those who have had good experience with clamps, but if you ask any good muffler shop, they will always advise the welding method.
I have had 4 different exhausts on my car and welding and cutting was never an issue for a professional installer.