Was the old one stuck open when you removed it? The most common cause of slow warming is a thermostat that is stuck open.
I'd say the old thermostat was the cause of any change in warm up. The temp rating should have no affect on warm up times, that's based on total coolant volume and outside temps.
I'm surprised to read of any noticeable mpg change, most people can't tell the difference.
I had to do my radiator last May, and mistakenly installed a new 160 T'stat during the project. I did all coolant hoses and the T'stat, attempted to reach the timing chain but broke a WP bolt, and backed up(plus new balancer). My 302 has run around 170-180 during last Summer, and this Winter was 132-140 most of the time. I put a bunch of miles on my truck delivering mail, with a scan gauge displaying important data constantly. The older 95-01's have very large radiators, so seeing low coolant temps is very possible. I didn't get any CEL codes last year, but when outside temps stayed below about 50, then it'd trigger a code maybe once a day. In freezing temps, it generally does it once going to work, and maybe once every three days on the route. Mine is an extreme case, I meant to use a 180 T'stat, which will not throw a code. I have a 180 in my primary mail truck, a 99 SOHC Explorer. It ran about 180-190 all the time, and there was no real change in mpg versus the 195 OEM T'stat.
There is no extra wear from a colder T'stat, that is a myth, so help make it an old myth. Mileage can be effected depending on use, cruising down the highway versus city traffic. I've seen no real change in my constant stop and go job use. I prefer a 180 T'stat, for the(also immeasurable) better power. With the 160 T'stat I have just enough heat in Winter for limited use, for a DD I'd prefer a little more heat.
