I'm a new Pecos owner as of Saturday. I've cooked with it exactly once, but was very happy with it.
I was looking for a cheap offset, and read tons of reviews and posts on this forum. Although all the COS models have their flaws, the Pecos seemed to have the fewest.
I did see lots of recommendations to "hand-pick" your smoker at the store, and I'd concur. The Academy I went to had about a dozen to pick from, and all but a couple had pretty obvious flaws (doors didn't seal tight, holes in the welds, loose wheels, etc.). They all seemed fixable, of course, but nothing I wanted to spend time on.
I burned it out at around 500* for an hour or so, wiped it down, then seasoned it with vegetable oil at about 300* for a couple more hours. So as not to waste a good fire, I threw on a couple racks of baby back ribs and some sausages. I was still learning, but was able to keep temps between 225 and 300 throughout, and everything came out great.
As for tuning plates, I wouldn't necessarily assume you'll need them. Disclaimer: I am not a competition cooker by any means, and the only thermometer I'm using is a Tel-Tru in the pre-drilled lower-right position. But I intentionally spaced out the sausage links to try to get a sense of hot spots, and they all cooked very similarly. I had a water pan close to the firebox, but otherwise the temps seemed to be reasonably even across the grate. I may do the biscuit test at some point, but more for the sake of knowing than for trying to fix. I'd worry tuning plates would just lower my temps across the board. I'm sure others will have better advice than I do there.
Overall, I was really happy with the Pecos, and it was easier to maintain temps than I thought it might be. I was burning good, dry post oak, with the smokestack vent and firebox door wide open. After I had a good bed of coals going, I needed to add a split about once an hour. When I was first getting the fire started (and had lots of white, billowy smoke), you could definitely see all the leaks around the doors. But honestly, I'm not sure that was a bad thing. I mean, I could seal those up, but if it were more efficient I think it would be really hard to keep the cook chamber around 250 (without me cutting my wood into smaller pieces, anyway). With a small cooker like this, I think a little inefficiency might be a good thing (even though you'll use more wood).
Hope this helps! Good luck with your purchase!