Not true.Not for us anyway. I have never ever seen any .bin file where a car stays in closed loop untill any where NEAR that tempeture.The ECT for closed loop on a stock 2000 GTP is about 50*F, and a stock 98 GTP is about 76.1*F. Even for corvette's ect...The cupple .bin files I have seen for them the Closed Loop ECT is no where near that. I can also promise you that by installing a 160 thermostat will not wash down your cylinders because of excessive fuel or colder block temps.
I would have to agree on the mpg loss. Even though I have not seen any concreate results from myself mostly because I have not been keeping a close eye on it.....