Yeh thats pretty much what I mean. Apart form how does that explain the fact that sometimes when you buy a stage 5 turbo lets say, the stage 3 turbo lets say gives more power than the 5 which I don't understand at all.
Off topic, but still its worth answering Just remember there is a thread with this stuff in it already but I will save you the time of reading through it.
With the stage turbo 1 it is small which means a small increase in BHP, (which you know) and the stage four is the biggest turbo hence the highest increase in BHP (which you also know. What you may not know is that the stage 5 isn't really stage five, its an original turbine kit and I will get back to this.
Now you may or may not know, with the small turbo it offers low power increase(bad) but has very little lag(good) (which means the turbo kicks in almost strait away when you hit the accelerator, so gives the BHP increase at low engine revs) which is good news this is good for tight tracks.
With your big stage 4 turbo it offers the most power increase(good) but has lots of lag(bad)(which means the turbo kicks in very late but when it does finally kick in at higher revs it will give a massive boost of BHP allowing you to have a higher potential top speed) this is good news for fast tracks with long straits.
Tracks good for small turbo:
Autumn Ring
Citta di Aria
Tsukuba
Deep Forest
Leguna Seca
Tracks good for big turbo:
Test track
High speed ring
Fuji (any era)
SuperSpeedway
Midfield
Now when you have a track, you decide how open or tight the track is and decide which turbo suits it (if in dought go for the bigger one its better in most cases thanks to the huge power output) so you choose 1-4 on which will suit the track best.
Now with the original turbine kit it allows you to have both(two turbo's i thinks one big one small). Instead of choosing between stage 1, 2, 3, and 4, it gives you the best of both worlds, the Low lag(good) of the Small turbo and the high power(good) of the big one (but not always as high power output as stage four). This is in essence the best turbo, good potential acceleration good potential top speed. So although it tends to have lower power output than stage four, I really wouldn't worry about it unless there is a big difference(as there is with a minority of cars). So you don't have to buy all turbo's, just buy the one, original turbo(stage 5). Even for fast tracks like Fuji the acceleration advantage the original Turbo gives will be better than the slightly higher power of stage 4 obsolete.
The only race i can think of where the slightly higher power will of stage 4 will be better suited than the overall standard of the original turbo, is the 'Like the Wind' Race in the expert hall, but for this you will be better off with a Group C race car or Le Mans Prototype car, and these cars don't have original Turbo's anyway only stage 4.
So there you have it, stick to original turbo's if your car can be tuned in the tuner village ignore stage 4 and all the other stage turbo's go for the Original turbine. However if you cannot tune your car in the tuner village like most cars, then follow the very simplified guide, about which turbo is the best for which track.
Note; not all people agree that the original turbo is the best one to go for, and prefer the Big stage 4, its just my opinion but one which works well for me.
Also a lot of these are assumptions, or have been simplified to make them easier to understand, but I would imagine they are fairly accurate.