Shawn,
I just went back and read that thread where you described your experience, including James Garney's response to the thread. I think Dave is right that it's very easy to over-blow this instrument. I've got the longer tuning slide pulled about a half inch from completely out at the moment, but that's better than when I started. I actually pushed it in a little today.
When moving from my marching euph to this thing, I liken it to stepping out of a Ford F-150 and into a Porsche 911 GT2 RS. The marching euph is powerful and hauls a ton of sound, but the Sinfonico... it's a sensitive, touchy tool that wants to be stroked, told it's pretty and handled like the lady it is. I can't just blast a lungful of air into it and expect to be anywhere near in tune. If you relax and sigh into it and be gentle, there's a "sweet spot" with every pitch where the instrument just RESONATES. I stepped outside and played it, finding the center of the tone, and it just echoed like I was pushing great lung-fuls of air through the marching euph. It didn't take a ton of air, either, it's actually really cool when I can manage to get it right. That's what I'm working on now, is practicing with it a lot at finding those perfectly supported pitches a little easier and more naturally.
I tell you, my embouchure, breathing and control are going to get WAY better as a result of using this instrument. I can already feel a difference just a few days in. I'm able to be more accurate with less effort. Now I just need to work on being CONSISTENTLY so.
I don't know if I agree with Mr Garney's comment that the instrument won't be good for amateurs. I'm nothing if not an amateur, albeit a moderately talented one, and it seems to be working well for me. That said, it's not an instrument that you just pick up and instantly start ripping through the last few pages of the Arban's book with perfect tone and intonation (I couldn't do that with any instrument I own, but the point stands). While there's a definite "center" for every pitch, it will allow you to EASILY bend a half-tone up or down, and will take your lead on where to go. So, if you're in a situation where you need to play something with a certain intonation to fit into a chord, for example, this instrument will let you do that without fighting you on it at ALL. For me, this results in ME playing the pitch off-center almost effortlessly (and frustratingly frequently). It's just... agile, and compliant, and much gentler than any other brass instrument I've ever played, and it's highlighting that I need to work harder to be a better player. That's NOT a bad thing.