Hi, most of my tunes are basics all round tunes. If you know what to do to adapt them to your own driving style or to a special track or a special kind of race ,like endurance races , you can do it, they are a very good starting point. But it's very easy to mess all the car too. So you can try to fix some things but you need to proceed with method , doing point by point ,few by few ,and coming back to the original base if it's not the direction you wanted to take initially .
For using the same tune for less power, you can do it if you stay with the same range of tires. LSD is made principally to contain the power and reactions of the car at accel and decel and it can still work this way ,from + to - HP.
For the gears ,you can still use the same ratios settings but with less power your car will have less max speed ,so you will set a lower speed staying with the same ratios settings ,only modulating the final gear.
if you use a car with differents Engine tuning stage ,the gears ratios will not match also. For example if you take a gears settings made for a car tune with ENGINE 3 , you will not able to set the same ratios for a gearbox of the same car with no engine stage. If it happen you can use the method that is at the beginning of my garage to set your own gears yourself.
In the other way generally it don't work . From less power car tune increasing too much power .
To resume, you can use the same tune if you stay with the same tires in a maximum 20 to 30 performance points range, doing some littles easy adjust.
Above this it can match but just sometime, and it's always better to have the good tune for the car you're driving. There is so much around here with so much good people doing cars, that it's not a problem to find one.
This link help a lot for sure to find anything you want.
https://sites.google.com/site/gt5tune/home
Enjoy your drive.