Interesting. Could this not have an impact by overheating by not circulating the coolant as quickly?
I too would be a little concerned not just with coolant flow but with charge rate as well. It could be that at idle the charge warning light comes on. During heavier electrical loads the alternator regulator will attempt to drive the alternator harder leading, maybe ironically, to greater belt loads. This can also lead them to make a noticeable whine until the rotor speed approaches at least 2500 rpm
Finally the smaller pulley will offer less traction to belt due to reduced surface area in contact with same leading to possibility of belt slip and increased wear.
If the new pulley is just a modest reduction you may get away with none of above issues but worth being wise to possible consequences.
