I think we are talking about different points. Mechanical grip is in a sense how much grip can you get for a given load on the tires. That load includes both the weight of the car and the aero based downforce.
The trade off we are seeing is the decision to choose (and I am making up numbers for illustration purposes) a suspension design that is say 3% less efficient when it comes to turning the normal load on the car (weight +aero) into lateral grip but the designers chose it because it allows for 4% more total load via an increase in aero based downforce.
F1 cars, can get away with running what is realistically VERY screwy suspension geometry in large part because so much of the real "suspension" action occurs in the tire sidewalls, not in what we would normally call the car's suspension.
I would say that the days of F1 on high profile tires are these days. The last groved tires were positively balloons in comparison to what Indy and other top open wheel and LMP cars are running. In part because so much of the suspension action occurs in the tires the F1 cars can run such screwy geometries.
Here was an interesting thread that at least started out on just this topic.
[ame=http://www.apexspeed.com/forums/showthread.php?t=38233]F1 front suspension geometry - ApexSpeed[/ame]
You certainly could move the front wings further forward and have more underbody aero without serious consequences. Indy and CART were that way for years. I understand not wanting to go back to the all underbody aero days but they seem to have go far in the other direction.
Nascar flying issues weren't about loosing underbody aero. They were about the shape of the upper body becoming winglike at 200mph. Underbody aero does have some advantages for racing. It's less sensitive to the cars around it. The wake problem as you get behind another car isn't as bad so it has the potential to produce closer racing.