Ive been thinkin the same as you matey,
Im after more power but I just love Celicas too much to sell it and Ive spent far too much modifing my GT to the point it is now. I would never get any of the money back.
As JO3 said Im in this for the long run now, Ill get rid of my Celica when I cant afford to run her or she dies. Im even hoping in the future that my Celi will be a weekend warrior tucked away until the sun shines.
Depends on what you wanna do. SC Celicas are scooby killers and puts over motorists in shock over how quick they are but you will never make the money back. Where as you can get something quick from standard and people will expect it to be quick.