Make a call to Kel-Tec. Sometimes they'll have them in stock even when the website says they don't. That's how I got mine. Midway was saying it was going to be a couple of months and I thought it wouldn't hurt to try. Five days later I had it in my hands.
If you even THINK you might buy a Sub, buy the extension now! It don't cost much when you buy it from KelTec (esp. when bought over the phone) and if you decide you don't need it, you should be able to sell it here or Ebay for what you got for it because KelTec always runs out.
I had mine weeks before I had a Sub. But part of that was that I needed it to make it CA legal. Even if it wasn't keeping me out of jail, I would still probably use it because my orangutang arms make shooting the non-modified sub nye impossible.