Franc I know you're a very busy guy, but one cool night before it starts to get crazy hot ouside, why not just take your car to a dragstrip and make a few passes to see what your car will do.
Everybody here will attest to your ablities to drive a manual shift car, so your track results should be considered a very reliable/credible test to show what a modified NA, XF can do with all the fixins.
Since the best times ever recordered by the Automotive Magazines back in the day were 14.7 sec. for a manual shift 04, that should be the base line to go by.
Personally, I think a car that weighs as much as ours, pushing 300 H.P. should be capable of almost breaking into the 13's (even with our 3.27 gears).
So far, nobody has been able to do it with an NA car on just the motor only. (without nitrous).
Maybe somebody in your neighborhood (who races an SRT6), could loan you some slicks. Then you should be able to see the 13's.
Remember, without photo's it never happened. But a timeslip will do too.