Perhaps marubozo might pop into this thread and offer his views/opinion. IIRC, he's running a SP300 and also has a SF to handle additional capacity needs at The Prized Pig.
I knew my ears were burning...
As mentioned, I run a SP 300 and a Shirley at my restaurant, and after being at it for about a year, I have experienced the highs and lows.
First and foremost, you need to know your wood even when running a gas-assist smoker, or else you could fark it up and put out nothing but baked meat. Learning how to keep that firebox stocked with the proper mix of seasoned woods is the difference between relying on gas or relying on wood.
I can get my SP cranking at 350 on just wood alone and never needing the gas. With careful fire management, I can keep it running for hours at 250-275 without the burner firing up once. Done right, it's essentially a stick burner with rotisserie racks and a convection fan. Done wrong, you've got jackasses who throw two little splits of wood in the box, set the temp on the controller, and walk away for three hours not realizing their wood is all burnt up and it's now just a big oven. Hence, the bad BBQ.
The Shirley is nothing short of amazing. With a competent pitmaster at the helm, it will turn our award-winning BBQ every time. I love mine, especially on smaller cuts like ribs and poultry where they can really take up the smoke.
But at the same time, it requires a lot of skill and a TON of wood to keep a big pit like that running. Maintaining consistent temps is an art, and learning the temp differences across the different positions on the racks is paramount to turning out consistent product. A rainy day? You're in for a bad time. Windy? That's another issue. Up here in Michigan you come into work in the morning and it's -10 degrees outside and you've got 12 hours to cook for 300 people? Good luck.
My bottom line is this. Without the SP, I wouldn't still be in business. That pit is already running with meat on it 24 hours a day. Even with the automation of a SP I'm still working 80 hours a week. Finding skilled people to maintain the Shirley for 24 hours a day would be next to impossible in this area. Not to mention cost-prohibitive.
At the end of the day, it's all about the pitmaster, not the pit. There are guys here in town who are turning out garbage on their stick burners, or at the very least, inconsistent product that's hurting their business. Just like the guys who are dropping $15k on a southern pride and can't make it work because they don't know how to use it.