Actually, I think the forum might have been Teamswift.net.
And the suggestions were...
Xtra Frugal injection
Xenophobe Financial independence
X-Files immobilizer
Xtraordinarily Frugal intentions
Xtra Fuel inside
Xample For insight (Honda)
Xtra Friggin inferior
Xtinguishes Female interest
But knowing it was a GM marketer who named the car, and knowing that the XFE designation actually means something on their Cobalt, etc., I'd be surprised if XFI doesn't have a real meaning too...