Coyote, speaking as the father of a 19 yr. old daughter (very much into cars).
You may just want to rethink that plan to give your son the SRT 6 when he's 16. 1st. it will be here before you know it, and 2nd. did you ever hear the old saying,"You gotta learn how to walk, before you can run?)
Now don't get me wrong, I'm all for keeping the XF for a long time, that's cool. But If my dad had handed me the keys to a '67 Vette, 427, 435 hp (that ran approx. the same 1/4 mile times as your SRT 6) when I was 16, In all honesty, I really don't know if I'd be here today to talk about it.
It is a nice thought though. Good luck with what ever you decide to do.

JCRUSE, Coyote gave you great advice, I'm sure how objectionable the ride is in an SRT 6 can be is directly related to where you live, and how smooth the roads are. People in Calif. and Fla. probably think everything's fine. Here in the Great Lakes region where the winter really destroys our roads it might be a different story. If you decide to buy the SRT 6, and it simply rides to rough for ya, then switching to the Limited springs would probably solve all your problems. I bet you could trade them with another XF Forum member in a heart beat. What ever you decide, I'm sure you would really enjoy the performance of the SRT.