I mean JC Spadons, you can download it from here
http://www.racedepartment.com/downloads/realfeel-ffb-project-cars-by-jc-spadon.11927/. Those are even better then Jack Spades. With those i think the GT3 feel pretty similar to the GT3 in AC (which i consider amongst the worst FFB from AC). I agree with your general sentiment about the less friction feeling, however those JC Spadon settings cure them to a certain extent. GT3 and LMP feel almost as good as AC..street cars feel like nothing really, and that is where AC shines. I stopped racing steet cars in Pcars.
As for AC - i don't really know what you mean with lacking "visceral sensation of grip". I think the feeling in AC is very grippy to be honest. Some cars like GT3 feel pretty boring, because not too much happening in the wheel (just like in Pcars), but for street cars, old race cars, or Formula cars, i think AC FFB is superb. The car control i have in those is better then in all other sims. Especially how well you feel the road is superb, every slightest bump.
In Pcars there is really no issue when you race "slower" cars, like cup cars or GT classes. I have had 30 hours in those now the last 3 weeks, and not a single time that i got divebombed or corner-cutted by the AI. They acted almost perfect, like you see it on a real race track. However if you get to faster classes like LMP or Formula 1, then the aggressiveness becomes unhealthy, because every slight mistake you make can result into a contact with AI, since they can't avoid you in time. AI didn't turn into my car however, like you stated it. No, didn't happen to me, unless i divebombed them in inside corner. in the GT3 class i think the AI is nearly perfect, best all around. I just stick to those classes where the AI is good, because it's a lot of fun. If i get to a class i see the AI can't cope with, or is to agressive, i just skip it, and have fun in the classes where it is good at. Raceroom has the same problem about overaggressive AI in Formula classes. You'd just have to code two different AI's, one for GT and one for single seater racing. Hopefully they stick to the Club/GT AI from Pcars 1, and code a new better one for the faster cars.
About AC AI, depends what you understand with challenging. They are definitely fast, but they are not "challenging". When i set the AI too high, they just drive away from me in front of me, but the one's behind still don't overtake me. I can assure you that i can drive with 100 AI for 20 laps, and make sure that not a single AI overtakes me. And not with unfair methods, just with proper defending. And that, while even 95 AI is too fast for me. That's not what i call challenging. Pcars or R3E AI is challenging, because they constantly put you under pressure, and try to overtake anytime you make a mistake. In AC they just back off when you retake the racing/inside line. Furthermore, you know that if you are going to make a mistake or break way too early, AI will hit you in R3E or Pcars (just like it would happen in real life), so you have to push all the way, while in AC you can drive a cozy race pace and be safe from behind. That's why i call it an overtake-simulator, and not a racing simulator, when it comes to single player.