I'd go through the following troubleshooting steps:
1 - Make sure your HSN is tuned well. There's a chance you're way lean on your HSN but that your overall engine temps appear OK because you're way rich on your LSN.
I'd get the car started (doing whatever the heck you have to do with idle screw and low speed needles) and run the car around at racing speeds for at least three minutes. Then start doing some high speed passes. *Richen* the HSN an hour at a time until you actually see (and more importantly hear) a fall-off in speed. Once you hear this fall-off lean it one hour. I'm guessing/hoping that what you'll find is that your HSN is too lean.
The only challenge in this step is that if you leave the car sitting long at all, it might load up and then when you do your high speed passes it may act rich at high speeds simply because there's residual fuel. For this reason one you start doing your high speed passes, try not to let the car sit much at all. And since it'll sit for a minute while you make your one-hour changes to the HSN, don't "count" the very next high speed pass. Rather do a few so you're confident there's no residual fuel.
2 - Because the theory I'm operating on is that your LSN is too rich, your next step is going to turn the idle screw counter-clockwise a bit so that the gap is even smaller and then compensate by leaning the LSN a bit. This will take some experimentation but when you get it right, the acceleration should be good and you shouldn't have the symptom where it high-idles and then drops down after 5 or 10 seconds. It's very common for the idle gap to be too large but the LSN is tuned rich enough to make the idle sound OK.
If I'm suggesting you do exactly what you've already tried then I apologize!