There are too many subjective things to consider when comparing ride and handling. The biggest one being the person driving the car. My advice would be to climb into a 2000-2002 STR (NOT a 2003-2005 STR) and see if there is a difference that you like. Until then, claiming the jag setup the best is meaningless. The only other option is to try it and gauge the results for yourself with the acceptance that it might be worse in your own opinion.
Should the jag set-up handle better? Presumably yes, but to what extent is unknown since the Jag and LS are not 100% identical platforms. The problem with just saying that it is better or the best is people will expect a night and day difference. In reality it could turn out to be an extra 0.01 G in the skidpad and 0.1 or 0.2 seconds through a slalom. If that cost you $1000, the question then becomes if the juice is worth the squeeze.
Since his scenario seems to be bang for the buck, I would still recommend investing in a decent set of performance springs like the H&R or Eibachs. That along might give him most of what he wants in the ride and handling department.