This is an interesting idea, I'll weigh in with my own experiences. I want to straight up say as others have, I recently bought a car, and I think it would've been completely mad of me to not drive it, and many other examples of it, in real life before deciding I wanted it/one like it, I had GT5 at the time.
Two cars in the game that are close to the cars I own, a Golf V GTI 5 door DSG - as a 3 door manual in game, which understeers way more, and is much harder to flick in game than in real life, but also has less wheel spin in the game, so I guess the game is out both positively and negatively in some respects, ultimately it isn't accurately behaving as my actual car does in real life, while it is still behaving as a FF does, so its tricky. Also, doesn't sound like it does in real life.
I also had a Peugeot 306 XT - in the game a 307 (virtually identical, same engine, ect), also not a good show in the game to real life, again in positive and negative ways.
Obviously gear change is completely out of the question in GT6, whether its speed, feel, smoothness, it just can't be tested in the game at all, for any car (accurately). Interiors, also virtually out of the question, as is practicality, real world economy, there are several ways this point blank won't help... But your thread isn't asking if people should 'only' use GT6, it's asking if it can aid you "AT ALL" (all caps, because 'urgent'?).
I think to answer your question though, yes, this can aid you, but it's probably more of a database type of aid rather than a driving simulator one, especially with the new search function... Then again, this data has been mapped well on a site called kudosprime, albeit GT6 has 3d pictures. Want to know if a Ford ST weighs more than a Golf R? Sure, it's got that (Golf is apparently 100kgs more, I'd double check that though). Visually too, comparing two cars standing next to each other on their looks is helpful.
I suppose to a certain extend the simulator is of use to a limited degree by quantifying the stats into base car types. How much slower is an ST than an R around a track? Probably similarly timed to their actual counterparts on the same real life tracks... But look up Jeremy Clarkson, seeing how fast he could get an NSX around Laguna Seca in a PS2 vs. RL for more on that.
Again, very interesting idea and perhaps my two cars aren't a wide enough pallet to accurately describe the GT5/6 engine, but if they are it would be as... wanting for reality.