Last time I flew NW into MCI, it was mainline to/from MSP and DTW (DC-9 IIRC). Not sure if it is today, but if it is, they you will get the upgrades.

Forget about the HP idea and buying space positive upgrades, fly NW MCI-MSP/DTW-PHX and get your free upgrades. The majority of MSP-PHX flights are 757s, so upgrades are almost a breeze with 22/24F seats. I've only once not gotten upgraded MSP-PHX on NW as a CO Gold in years, and that was an A320. Odds are pretty good.