I use TT as a base for my setups, but still try to see what happens in a race situation in comparison, though I don’t always have the time. It has worked for me so far, although maybe I should try using a full race weekend, and see that guides the development of my setup.
I definitely think that Codemasters should implement a game mode which allows a player to develop their setup, with fuel and tyre simulation on. Pretty much a testing game mode. It would be so much easier :P