Here's how i understand ti all works:
Balanced / Intermediate / Aggressive
Selecting this effects the order you get your upgrades. In aggressive mode you get the more aggressive updates first. E.g in season 2 in aggressive i didn't get 'cruise' mode for my engine til Hockenheim.
Where as in season 1 on balanced i got it much earlier. Also on aggressive R&D got off to a VERY slow start, didn't get anything til i hit Europe races really.
Upgrades and R&D
You have EXACTLY the same upgrades from season to season, there is NO difference, except maybe in the order you get them (
so after 4 seasons you DON'T have 80% extra downforce sorry).
Tiers for performance
What effects the cars performance from season to season is the Tier it is in.
There are 5 tiers for each car.
1 being the best, 5 the worst
Each car has different values for each tier. Which changes the handling and engine power.
So what is actually happening is your car is moving up a tier, and getting better handling and power. You still have exactly the same upgrades to bolt on during the season and improve the car WITHIN it's current tier but they don't carry over.
In effect your just doing the 2010 season 7 times in a 7 season career.
Here's a nice chart by Steve Stoop that explains it all.
http://www.racedepartment.com/f1-2010-game/52057-do-new-teams-improve-each-season.html
You will notice that Lotus is the best starting car, and you can also get it very competitive in tier 1, your only 5 horse power behind the big 3!
Red Bull are down on engine power once they drop out of tier 1.
Force India have great horse power in all tiers (shame bout the pit bug for em)