Hi Phil!
Thank you for sharing tips.
I have recently started an article about speeding X-Cart up:
http://kb.x-cart.com/display/XDD/Speeding+up+your+store
You can find useful info there as well.
As for your findings, do I understand you correctly that Optimize website option (point 3) had the biggest impact on load speed? How would you measure effect of points 1, 2? How much speed did they give to you?
Tony.