who has had a muffler shop weld in their subframe connectors?

Yep... same here. $50 to have them MIG welded. Buy yourself one or two cans of undercoating to spray over then once they're in. If you get the connectors that have the two bolt holes then you're golden. Tighten up the bolts to synch them into position then weld away. Make sure that the suspension is loaded when you weld them. Don't let the muffler shop weld them in with the car on a body lift. Use a lift that leaves the car sitting on the wheels.
 
Steeda full lengths, ( along with the rest of it Steeda suspension :) ).
welded it myself, wonderful.
Undercoated them as well as the rest of the undercarriage while i was at it.

While your at it, may want to look into seat braces. They weld to the subframe connector and where the seat bolts to the pan to increase strngth :) Good luck