For $1000 you can get a new WBS IC and a downpipe.
You could also hunt the classifieds and get them for cheaper, and go with headers instead.
There's a good chance that with you running the absolute piss out of your car that there is some damage. What are the plugs gapped at? No need to go to a cooler heat range on a near stock setup.
As far as the thermostat, I agree that keeping the 195* in there is the best.
Could be spark blow out, with the combustion chamber staying so hot for such a long time it's possible. You could try closing the gap down and see if that helps (Though I don't condone taking a poor handling car with tires that aren't properly rated for that speed to those speeds)
Usually a clogged catalytic wont let you accelerate past a certain RPM, not usually a specific speed.
it could ALSO be the PCM trying desperately to save your **** by pulling spark and fuel to keep you from blowing up.
The SES flashing means there's a continuous misfire condition. Swap plugs or wires and see if the misfire follows it. No need to go fancy on the plug wires or spark plugs, though you can if you want. (A set of OEM plug wires and some AL606's will be fine)
One other note to keep in mind, our cars aren't meant for sustained high RPM operation. You wont be getting proper lubrication throughout the motor doing long high speed runs. Just food for thought.