I've started this new thread to avoid "Highjacking" the original thread.
First of all, let me state that my preference for downshears extends only to shallow grooves and dados. Anything deeper than 1/2 demands an upshear.
Chip evacuation is a non issue in anything less than 1/2" deep. Run an upshear and a downshear side by side in shallow grooves and there is no more waste left behind in the downshear slot than the upshear.
I've been using spiral bits for more than 7 years, since long before they were available to the general market. I was introduced to them by Ross Gobble of Onsrud. I've used them in CNCs and hand routers.
Typically, I keep an upshear on hand for mortising, a downshear for dados and grooves, and a compression for cutting through.
Heat is certainly an issue as you state, but again, my experience is that chip evacuation is perfectly adequate in shallow grooves.
Please understand, I post this to further the technical discussion, not to be argumentative.