Does having the tires siped by a company like Discount Tire do any good offroad or in snow?

It depends on the type of tire and how much tread is on it. Most all terrain tires already have siping in them from the factory where a Mud terrain does not.

Example: A 1/2 worn swamper is a perfect canidate for siping.

However a new Swamper may not be depending on your type of driving. Siping a tire with over 1/2 an inch of tread can cause tread squirm. This is when a tread  block does not hold its shape and folds over actually causing a loss of traction.

Another tip. when siping a swamper or other mud terrain type tire only sipe the center tread patch. This will reduce chunking the tire and decrease tread squirm.

Mike
 

What about some Cooper Discoverer STT's with 1/4 of the tread left (http://www.kielbassa.com/tracker/assets/newtires/tread_closeup_01.jpg)

Not worth the extra money. To near of being out of life. but thats me.

F.Y.I.
Some sales people say that siping makes your tire last longer.  (maybe on highway only tires)

Every type of tire I've had siped seams to wear faster then if I had left it alone.  The little tread blocks created by siping, tear, break, get worn out faster because there isn't enough rubber around them to protect itself.  (think smaller lugs are more vulnerable to tearing and breaking)

But the traction advantage on ice and wet roads was noticable.

~Nate