Phil,
I bought my SVT new in Feb '99 and within a week replaced the stock air filter w/ the K&N drop in. I didn't notice any increase in throttle response, although I was still getting used to the car so if it was there it was mild.
If you go with an open element K&N filter you will get the 'smidge of throttle response' (what is a smidge?) you spoke of.
I have open element filters on both of my cars and installed one on my girlfriend's explorer, increased throttle response in all three cases. You may see two 'smidges' of increase at WOT!
