Well i fancied a bt of a quck race last night on Forza 3.  Racing in my nice shiny Mazda Furai.  All in all I put in some nice lap times, and pulled off some decent over-taking, but overall I tended to come last every race, at least after the people behind me logged out.  
 
This prompted me to look at the cars people are driving, interestingly - everybody except me drove the same vehicle - the Peugeot 908, tuned up to PI 999 class X.  Now I had a test-drive in one, nice and fast, sure but... Well, in reality - I can't see any valid arguement for racing anythng other than a maxed out Peugeot 908 - which is a bit silly, and a bit boring?  What's the point of havng a hundred cars to pick from if everyone has to pick the same one to compete?
 
I don't know what the solution here is, it isn't a 2D fighting game which requires 'balanced' characters, but it is an interesting issue with Forza 3 - I suspect this issue isn't replicated in Drift, where there s probably more flexibility and car and setup to suit drivng style and what not - but even so!
 
I think there are some poor decisions in Forza 3, the fact that some cars are rendered pointless by the PI class.  Take my Mazda Furai - I bought it cuz I love the car - it starts out pretty in the middle of R2 at about 850, now turbo it and it becomes 915, 916 if you fit lighter, bigger alloys... So you probably can't compete in R2, not with a good driver in an 899 car, and you can't compete in R1 probably not anyway... Which is annoying.
 
Whether these issues in GT5 exist I don't know, I suppose they are the sort of problems you get when tryng to make a fun, balanced game and realistic simulation. 

Having just rebought  the ultimate edition i've been trying out all the new cars from the DLC and its sad to say that i'm just playing it offline for now, i had the same experience as you last year when i was big into racing online and i was disappointed that rather than setup races with balanced mix of cars everyone just drives a rocket powered car that handles like a brick but smokes all the others in the straights, it left me underwhelmed but luckily i was able to get a couple of race weekends setup with Justin Calvert from Gamespot and a bunch of users from the site and because everyone had the same car it was tons of fun.  
i do hope they make balanced hoppers for the next iteration.