For fuel efficecny you just basically need to shift gears before the turbo spools or you get deep into. As far as changing for spirited driving, as long as you arent slamming gears or dropping the clutc between you should be relatively fine. As far as keeping your speed and boost up its usually before redline. That point at where your boost is highest/before it starts to taper, us usually set upon in the tune. You just need to practice and pay attention to your ca and your driving habits to figure most of this out.

 

In regards to racing someone, I'll usually shift as close to redline as possible without hitting the rev limiter. Typically around 7200 for my little 2.0. Not sure what the redline is on the 2.5s.

As newtothegame stated, as long as you aren't slamming gears or dumping the clutch you should be fine.

Daily driving, I usually shift around 3500 - 4000 rpms. I've been getting 23mpg with the VF34 and larger injectors when I do that. Problem is, I love boost. So my average mpg is only 19 - 20.

 

The torque curve of your car determines the optimum shift points. Basically you shift when the next gear is making more torque at a given RPM than the gear you're currently in. On the 2.5L, this is ~5500 rpms and on the 2.0L it's ~6000rpms mattering on how it's tuned. Running it to redline or beyond is just putting extra ware on the engine and costing you time.

 

using this as a basis, you can "shorten" your shifts to save gas (shift lower than 5500rpms for example). Take this concept far enough and you'll be gleefully putting around like a non-turbo Impreza