A good sound system is the way to go! It really makes a difference in the overall experience. And much like everything else, if it fits, it sits!
GTR2 can be loaded with different modifications, so the list of tracks, cars, liveries, sounds, tweaks, etc. It never ends! We still have racing leagues competing today. Super fun if you're competing with the right group. The game features two practice sessions, two qualifying sessions and then the final race. There's day/night and weather for every track, so the experience is always changing. I've never raced around Laguna Seca at night before (It's my favorite track to compete in IRL) but I love doing it in GTR2, haha. Little stuff, like how much fuel you carry with you, effects the weight, and also where your tank is located effects balance. Driving on grass areas on a wet track can make your tires muddy, effecting their grip even further. Little details such as those blew every other racing game out of the water back in 2004/2005. Meanwhile, everyone was praising GT4....great game, no doubt, but it was nowhere close to GTR's level of detail and realism. But, I digress....
So you see what I mean with the choppiness? The cars sound....artificial. Like you said, the sound is just a latter of high and low revs. It's not smooth...continues...varied. That trailer of pCars you mentioned has awesome sound...especially that Zonda! But it doesn't really sound like that in gameplay...maybe they haven't released the full sound update for the beta testers? No idea...
pCars, the game itself, feels a lot like the successor to GTR2. In fact, SMS recruited a bunch of staff from SimBin to work on the title. I'm super excited, cause I've been waiting for a new GTR game for 10 years, haha. No idea when SimBin is releasing their jewel, but knowing them as perfectionists, they aren't releasing anything until they are 100% satisfied.
Oh God, don't get me started on Shift 2....what a hunk of junk. First of all, what the freak is up with the rear wheel steering in that game? Have you ever tried driving from the chase cam? You see the back end stepping out on every turn and then aligning back up again with no effort. Not to mention the fact that the cars feel like they are constantly being driven on ice. There was so much understeer and powersliding in that game, I couldn't even put 5 hours into it. I went back to it when they released a DLC pack (It had a Datsun 240Z, I believe a couple of Porsches, a GT40 and some other cars) hoping the game would be fun. Nope, still sucked.
In regards to the MP4-12C...it sounds better in FM5 IMO, but it doesn't have that dirty growl. No cars do, in fact. That's what I was mentioning earlier. D3 has those sounds. The cars in FM5 sound too....civilized.