I have two 22" conversions on a Performer and OTG platforms. I also have a big bullet.
(and test driving a Mini conversion)
The Cajun Bandit gets used more than the WSM. Do a search for any of my CB posts.
I am kind of biased but for different reasons-I bought the kettle conversion so I could use my Weber or EZ-Que rotisserie rings. It wasn't possible to do that on the big bullet, until C&C Grilling Co. introduced their WSM/Kettle rotis ring. By then I had modified the center section of my 22" WSM into a giant rotis ring.
So....I now have a 22" WSM, a modified center, and the Cajun Bandit ring, so I could do what I wanted to in the first place, but still use the kettle conversion out of convenience - I like the fact I can use the kettle lid with out the center section-something the WSM is not made for.
I do a lot of rib and chicken cooks on mine using the Rib-O-Lator, EZ-Que cradle and Char-Broil baskets, so my longest cooks have been under 6 hours, so I can't personally claim to be an expert on the long cooks, but I have all the confidence you should have no problem maintaining temps yourself. If you want peace of mind-add a Guru adapter and set it and forget it! It's next on my list.
As far as using a Bar-B-Kettle or OTS/OTG, I was concerned at first about ash clogging the daisy vents, so I modded out an old kettle and placed WSM vents at the charcoal grate like the WSM and found I had too much airflow and ditched it in favor of the OTS/OTG. There is a bit of a learning curve to find out your vent settings, and I found it much easier to do using the OTG over the Silver, but its not that big of a deal, because it does not take much to keep it going.
Either way you go the guys at C&C have you covered whether you get the WSM or have a kettle.:doh: