You are right Aaron,
The O2 sensor just needs to heat up to "Operating" Temp....It does not calibrate only according to the temp of the exhaust.... I was just stating that hotter exhaust = a lean mixture and cooler exhaust = a rich mixture to a post left by someone else. I know this from toying around with Turbo cars for awhile.

So don't worry about the "Temp" with the spacers it will certainly get to its proper operating temp and function properly. It just wont see as much O2.