User blog comment:Sirebel/Mazda Update (5.4.0)/@comment-95.97.99.242-20170914093202/@comment-26829117-20170916140240

My approach isn't much different to yours. I don't normally fully upgrade all the cars but in this update but three had to be fully upgraded for career and the fourth was in WTTT or a top speed event (I forget which). My order is:
 * Win cars, sometime minimum gold, sometimes just upgrade for career
 * Upgrade for 100% complete
 * Upgrade for WTTT
 * Upgrade fully

With update 5.5 I don't think I'll be doing the last step, it's way too expensive.