7/8" is standard bar size for all bars. Oversized bars are 1-1/8" in diameter, in the middle at least, and 7/8" on the ends.
Those "crappy stock Renthals" you had are the 7/8" Renthals you mentioned; they are standard equipment on several bikes now.
That $350 combo was a new top triple clamp. On a CR, you can change just the bar mounts to go to oversized bars; I'm not sure if that is an option on a YZ. If so, new bar mounts can be had for $50 or less. I went with an RG3 top triple clamp and Renthal TwinWalls on my CR250.