When I first read the title of this thread..I saw thought to myself:
"two superchargers?! Like the imfamous Aston Martin Vantage 600"
Of course, I see that it doesn't. Shame

Definitely not surprising, although that still means the new XF will be far behind vehicles like the M5 and E63 AMG.
Back in 2002 Jaguar's S-Type R was developing 400PS, therefore setting it's self in a league with the E39 M5 (400PS) and whatnot (power at least) Don't get me wrong, 420PS is a great deal of power, but the E63 and M5, as well as the upcoming RS6 will blow it to smithereens.
EDIT: Hopefully the XF-R takes care of that!
Perhaps Jaguar's motives are not to compete with any of those vehicle. Anybody know when they are officially going to release it?