You can't go buy what the Tamiya and Xray guys use, I know when I ran a X the rear bar was out, but on the S411 I will run the 1.2 or even the 1.4 bar, the later which help the car react on corner off and not roll as bad or flattens the car out. Without the rear bar the car feels sloppy loose in and out of the corners.
I would also look at a different body, like a Speed6, as the Stratus once was a good body but lacks overall stability compared to a 6, a LTC-R might be a good choice as well, but can be a little more aggressive in the front, so a proven body is the Speed6.
If your too soft in the back you'll just overheat the tires too soon and that will make it loose again.
What are you running for lower roll center shims? On the CRC carpet with SXT I would run a minimum of .5 but probably the 1mm shims front and back. Also I would run 1000cst in the diff, 2 gear.
The rear link (in/in on the holes) are going to make the car have a aggressive side bite, which will cause a heat issue again. I would try the long link, outside hole on the hub and inside hole on the bulkhead. For spacers from the rear inner link, outside hub and front inner, I like 3/3/4 or sometimes 4/3/4, but never seem to vary much from there.
Use the V2 rack and run the 2mm of ackerman.
The 2 pce motor mount just about always requires the setscrew in the center or you will do the spurs in. If you run the 1 pce mount this isn't a issue, although the 1 pce sometimes feels like it gives the car a torque steer feel sometimes. You can run the 2 pce with the setscrew and still have a modified stock top deck. I have modified one by cutting the rear center section along the long bar on the right side, still retaining the hole for the motor mount, being attached to the left side long bar. This still gives the flex you want and doesn't let the motor mount flex up which is what takes out the spur gear. I tested this mod for 2 race days now with no gear failures and a slightly more flexi car.
I really find that if everyone is running SXT on the track, to stay consistent you have to clean your tires in between runs. Your right that you'll feel no difference in the length of time it sits on the tire, but cleaning seems to make it the same every run. I use motor spray and wipe each tire, then put on the SXT, time not really a factor.