Virgin doesn't have KERS, the 3 back end teams should all be pretty similar though and I think out of whatever one you choose, if you put it on legendary and have longer races, you will still be fighting for position among that group. As I found in the HRT on Pro I was struggling to get away from them at times, yet as soon as I joined Williams I was at the top. Recently been doing 100% races with my friend on Legendary and I notice some laps you will get pushed and possibly beaten, but other laps you will close in on them and it varies (also depends on how good you are at the track) i.e. I got pole at Kuala Lumpar, made some mistakes but ended up finishing 2nd in the Lotus-Renault and I love the track, but only got pole by .300ths and at the start of the race I was really pushed to the limit as my car didn't cope with fuel load as well.
I'd guess the 3 'newer' teams would be pretty equal in the game though. But I would suggest making it harder for yourself to make it much more enjoyable. 20% races just don't seem as fun!