There really isn't ANY direct competitor to the Panamera other than the MB CLS, the other 4 door 4 seat driver's car.
Consider the pricing here in Canada,
CLS550 @ 93k, Panamera 4S @ around 100k+ options = around 120k
CLS63 @128k, Panamera Turbo @ around 120k+ options = around $140k
If we includes the 5 seater performance driver's cars like M5, S6 and even Maserati Quattroporte
M5 @ 106k
S6 @100k
Quattroporte @ 108k
In Canada here at least, the Cayenne + 5% pricing is already at a premium over the competitors.
Somehow I don't think Porsche is gunning for the S-class, 7, A8 with the Panamera. For one Panamera doesn't have enough gadget to be consider in the same league, #2, those big limos just don't have the handling needed to be a driver's sedan. #3, Panamera is missing that 5th seat, and it's low rear roof means the owners can't be chauffeur around, same thing like the CLS but unlike the big sedans.