Really? I have never heard that. My 45 year old YM2310 starts quickly at 40 degrees without using the thermostart. Below 40 I use the thermostart and it might make a little difference. But then I have been through 3 thermostarts that I bought new. The first 2 didn't work after the first use. The third might be working, at least it worked before I screwed it in. In any case the lowest temp I have started my old Yanmar was about 30 degrees F, and it started pretty fast. Smoked quite a bit though.
Eric