Hmm... not convinced.
A ride at -20
oC is a lot harder than one at 2
oC. Ice excepted (please no-one start that crap again).
A ride at 20
oC is going to feel a lot easier than the same one at 40
oC.
You can visualise what the graph looks like. Given it's rising on the left hand side and falling on the right hand side it's got to plateau at some point between the two.
I don't think there's much point arguing about an exact figure given the subjective nature of 'ease of ride', individuals are vastly different. I can cope with vast ranges of temperatures with either shorts/ss-jersey (riding South towards Eskdalemuir in the maelstrom) and rides like the Severn Across where I didn't add or remove a layer during the entire ride despite it going from near freezing to 15
oC and back down to near freezing over the 24 hours; my leggings, leg warmers, base layer, long sleeve jersey and jacket stayed on the entire time (I seem to remember you commenting about this at Stow-In-The-Wold).