My 2009 is a 5 speed auto.
The reason for the throttle position, gear selection and vehicle speed requirements for the lockup is to ensure that the engine doesn't suffer any strain while the torque converter is locked.
There are occasions when I wish I could manually lock the torque converter, because it does get frustrating to be cruising along at 90km/h (actual speed) and you have to use a little too much throttle, resulting in the TCC unlocking.
At that speed it seems to save about 500rpm. On ideal figures (these are not necessarily accurate figures but they'll be a fair guide) the vehicle should use about 0.0417ml per injection per cylinder (at 10LPHK, so unladen traveling). If you increase the RPM to 2500 for the same speed and duration, your LPHK increases to 12.5 immediately - just for a 500rpm increase. It will actually increase more, because now the engine/gearbox isn't linked, there's more throttle being used so the injector duration is slightly longer.
Here's how I calculated the fuel used per injection:
At 100km/h, engine rpm = 2000 revolutions per minute
There are 2 power strokes per revolution in a 4 cylinder engine (one cylinder gets either a power stroke or an exhaust stroke per revolution, or 0.5 power strokes per rev, times 4 cylinders = 2 power strokes per revolution).
This means 240,000 power strokes per hour (at 2,000rpm) so 10 litres divided by 240,000 is 0.0000417 litres or 0.0417ml.
At 2500rpm, times 2 power strokes per rev times 60 (minutes in an hour) times 0.0000417 gives you 12.5 litres used per hour (100km/h).
The nutshell is, basically, that for just 500rpm more, you're using 2.5 litres per hour more, so the moral becomes - if you can engage that torque converter lockup, let it happen.
Caveat on the calculations: you can't figure it for 1500rpm, because at 1500rpm you're traveling slower than 100km/h, so the calculation necessarily changes.
Further caveat: these figures are based on an unladen average. Laden, the figures will change. The instantaneous figures (taken at any point in time with, say, a ScanGauge) will show different figures.