4000-5000 rpm shift point.

Anything less will make you press the pedal more because of the lack of acceleration just to stay in traffic. Anything more and you are getting into opening the secondaries and requiring more gas.
Yes, having a weak motor can cost you more mileage some times then having a strong one and just not using all of its potential.

Former owner of '99 CSVT - Silver #222/2760 356/334 wHP/TQ at 10psi on pump gas! See My Mods '05 Volvo S40 Turbo 5 AWD with 6spd, Passion Red '06 Mazda5 Touring, 5spd,MTX, Black