About 1-2 months ago, I cut out my secondary cat (replaced with straight pipe) and resonator (replaced with x-pipe). Last week I started noticing some odd exhaust noise. This morning it was really bad, so I took it back to the shop that did the work. They told me that the vibrations had loosened up one of the sleeve clamps. They tightened it back up, but then suggested that I bring it back for them to weld instead (at no cost). Told me the exact same issue happened recently with another C63 they worked on. This is a high-end shop with a good reputation, so I don't think they're doing shotty work, but it still seems odd that there would be a problem, given that it's my understanding that clamps are what are used on the factory exhaust. I would prefer not to weld, as it will make it harder to go back to stock, but obviously don't want to keep having this problem. Thoughts?

Pretty simple really.

Want to be able to go back easily? Don't weld.

Don't want to have to go in to a shop and have the bolts tightened everyone so often? Weld.

Just depends which you want more or less

clamp

You might have the clamps tightened and put two small, short tack welds 180º apart that can easily be knocked off for future disassembly. That's what I'd do anyway.

Maybe some lock washers would work? I don't know. I did mine with a sleeve clamp about 3 months ago and no issues so far. Are you ever going to take it off? Might as well weld it if they're offering it for free. I opted not to weld mine because I didn't want to drop the header and the sleeve was just so easy.

Use the clamps and have them tack weld as stated above. Best of both worlds.

You will have to go back to the shops to retighten the clamps after a few heat cycles. The clamps don't get loose once it has been retightened afterwards. My long tube clamps were retightened 3 times, and I have not had any issues. My headers have been on for almost 2 years.

I had the same issue.
Replaced the stretched clamps with new (OEM) clamps; All good.
Maybe have them checked after a couple of weeks.