Don't know what size wheels you have, but 114 ft for an IS350C with the 18 in Bridgestone turanzas. (source: motor trend http://www.motortrend.com/roadtests/...son/specs.html
Per Lexus, curb weight is about 65 lbs less for the IS250C vs 350C, so distance may be slightly less, although using that philosophy a passenger would sway the results too.
Tires would make a bigger difference. I didn't like the stock Bridgestone all-season 18's at all compared to my Michelin PS2s (which I had on my sedan, then put on my vert before upgrading to 19s)