I know I'm about 3 pages too late on explaining this, but I thought I would add my quick interpretation of the StageV turbo.
First off, doesn't the game explain that it's a step past the Stage IV turbo, but that it has the same overall boost as the Stage IV, with more low end horsepower? And I haven't checked in a while, but I believe it's also OEM equipment that's been modified, and would thus still only be a Stage I turbo with high boost, which would explain why it's got all that low end power.
But that's just my interpretation of the equipment designation. That it's not a bigger aftermarket turbo making all that power (because bigger turbos require more time to spool up), but a modified smaller turbo. In that regard, it would be quite superior to a larger turbo in eveyr way except CFM numbers.
But the point of my rant was to explain why you would still pay more for a turbo that seemingly does less (Stage IV). When I build a racing two-stroke motor for someone, be it Kart, Jetskiing, or dirtbiking, one of the main considerations with that motor is how peaky they want the powerband and how high up they want it. ALL motors build power in an escalating fashion, that is, power increases with RPM's, until volumetric efficiency collapses and power drops off - this is represented as the "peak".
Why would you want a peakier motor over a smoother motor? Often times with a professional racer, they are better at using the motor in high RPM's than a novice racer, as well as being able to better use the steeper powerband that accompanies peaky engines. As well, those said motors tend to make more peak horsepower, though not more average horsepower. Motors that have a lot of low end horsepower and less high end horsepower feel "squashy" to a racer; meaning when they mash the gas, it doesn't seem to get faster, even though their motor makes more average horsepower and is much more controllable than the peaky pro motor. The reason lies in the difference in power output between a given set of RPM's - on a violent, peaky motor the difference is great, while the smoother motor is less.
Make sense?
In short, the peakier motor makes more horsepower; as well, a pro can actually use that power "surge" to his advantage, rather than the feeling of sluggishness. However, a smoother motor will be more controlled for a beginner or tight track, and be less tiring.
I know these are real-world analogies and comparisons, but GT4 is so realistic that it would be a sin to believe that these principles don't apply to the game.