Sounds like the transmission is operating just fine. The operation described sound perfectly normal. The TCM keeps the engine at somewhat a constant rpm before locking up. I do agree that lockup is occurring pretty late at 45mph. A lower lockup speed would be more desirable, such as 40mph (like more powerful GM transmissions).

For one, I really doubt locking up the torque converter earlier would result in significant fuel economy gains. In the city, where the speed limit is closer to 35mph, the torque converter would still hardly ever be locked up. The power delivery of the car would be less predictable and "drivability" would suffer.

Second, the engine's FE is probably best when the rpm is between a certain range of approximately 1000rpm. My guess is between 2500 and 3600 rpm (Peak torque is at 3600rpm 1st Gen Aveo). Then it would make sense to try to keep the engines rpms within that range by controlling gear change and the torque converter lockup to get the best FE.

A good way to understand and get a grip on what I'm saying is to study CVTs (continuously variable transmissions). The reason why they are getting a lot more popular is because it's easier to control engine's rpms with them. I work with them everyday, not in automobiles unfortunately but in snowmobiles and quads. But the same theory applies.