in the last coupla years, i've ridden my bike plenty of times in 100+ weather. even ran a brand new engine from costa mesa to seal beach and back (about 75 miles round trip) at full throttle and about 90 degrees.
i figure the faster i go, the cooler the engine. gets more air-cooling, right?
honestly, i've never had a bike run poorly in the heat. it usually handles it better than i do.
another side of the coin is running regular oil and a rich mixture on a hot day. it seems to me it would have a slower cooling rate due to the "thickness" of the mix, as it would be circulating at a slower rate, not to mention the "molasses" effect as the heat burnt it up.
not exactly scientific, but it sounds plausible to me.