Thanks for the replies everyone, I do appreciate it!
I've still been fighting getting things completely dialed in, so I took the opportunity to go back to basics a bit. First I thoroughly cleaned my smoker - I cleaned the cooking chamber when I installed the S.S., but this time I went ahead and pulled the auger out and vacuumed everything out. Everything was fine there, but it's good to do this every couple of years anyway. I also cut out the auger limiting rod - which is more for high temp grilling, but I did this on the old Traeger 070 (at the suggestion of my ex-bro-in-law who worked for Traeger at the time). I also put together a dust vacuum contraption, which I had always meant to do, but never gotten around to - I'll post that in a separate thread. I even picked up a different brand of pellets - I've been burning Pit Boss because they are cheap at Walmart, but I picked up a couple bags of Lumberjack which I was able to find locally (but a bit expensive and no choice on wood mix).
Long story short (too late!) none of this made any difference and I was still having some difficulties getting the S.S. dialed in where I want it. I think where I've ended up is that my little XT is just not stable at all in a certain band around 225 - the pellet feed is either too low and it flames out, or just enough that temps increase, and it's no surprise when I really think about it that the controller can't cope with this. I spent a couple of hours playing around with manual mode and even I can't keep the temp at 225.
So, I've finally figured that if the smoker isn't stable at 225, I just won't cook there! This was important since I typically use 225 a lot, but I realized, eventually (remember, I admit I'm hard headed!), that the best course of action is to simple adjust my cooking range to where the smoker is stable.
The S.S. has no trouble keeping things at 175-185 which is perfect (this is the most important to me). And it's perfectly stable above 250. Last night I tried the go-grill feature (100% feed rate) and was amazed how hot it got - the stock controller had a lot of trouble getting about 350. Although on the XT you have to be very careful not to go above 400, and not long at these temps or you'll get a hopper fire.
So anyway, there's a long update on this with the hope that it helps someone else out there! And just to reiterate that my issues all have to do with the XT, and not the Savannah Stoker controller.