Just installed some Fabspeed tips on a 997.2S. Does anyone know how tight they're supposed to be attached? If I use medium force, I can still tilt them up/down relative to the pipe.

They should not be loose. I've heard of a few people that have issues with other tips. You may need to shim them. Check with Fabspeed.

the problem is, the tips clamp are not the correct diameter to clamp down on the exhaust tube

I used the stock shim, and I screwed down bolt/nut clamp within a couple of millimeters of maximum (I think the bolt started bending a little). The tips don't seem like they'd move on their own, but I can definitely move them up/down a few degrees without too much force.

You might try picking up a couple of muffler clamps made from stainless. A pack of two from Pepboys is about $6.00. Cut the drive from the end and adjust the length to just fit around the outside of the muffler tip. Wrap the piece around the outside of the muffler tip, slide the tip on and tighten the clamp. Hopefully that will take up enough slack. Of course you could also try cutting a few more slits in the tip to allow for a bit more compression.

Just go to a muffler shop and have them tack weld a couple of spots.

Only if you like the idea of replacing 400$ tips