Weal, the the 80′s concept benefited mainly from Turbo’s, teams could keep the turbos spooled up to maintain a stable exhaust flow to the exhaust. High reving engines just arn’t that stable, downforce cuts as the throttle comes off, right at the second you need it.
The last exhaust diffuser car, a Newey creation, the Mp4-18 was great for a few laps, then it set itself on fire, an it failed crash tests n stuff, I have the sneaking suspicion that a few mp4 18 ideas which technology hadn’t caught up with yet are on the RB6.
Also diffusers still suck the cars to the ground by creating low preassure under the car, opposite of the rear wing which pushes the car into the ground. Air is forced to exit and disperse at increased velocity from the diffuser, lowering the preassure under the car.