How about this fine example just posted last night?
I've known people have the S-Type R's from new, and do have a soft spot for them, despite some reliability issues he encountered.
In a straight line with your mods, I'm not sure there would be much difference. The IS-F does have a better gearbox, so that could well give it a good advantage.
Around corners and general handling, it would be like night and day. I suggest taking one for a test drive if you havn't already.
XFR is going to be similar to the IS-F, but I would rate Lexus reliability higher than the Jag. I know a few people who have had various issues with the standard XF. I did a tour around scotland with one in a group of cars (some over 20 years old) and it was the only one to break down!