These are very different cars. Neither 302S/R have any interior; they're built from bodies in white. Even with a full roll cage, they weigh less than 3,300 pounds. The 302R has a seam-welded body. Though they use a Boss 302 engine, the electronics are calibrated to run on high-octane fuel. That alone must given them another 30+ horsepower over the street Boss 302.
However, your point is valid. I was getting ready to pull the trigger on a 302S as a track-day car. I now have a provisional order in on the Z/28. I think that the power/torque advantage, brakes, and massive rubber might gain back a lot of what the car will lose to the Boss 302S because of weight. At certain tracks I think the Z/28 will be faster.
Frankly, I'd love it if Chevy built a track-only version. Strip the interior more, add a roll cage, and bump the power. Even without that, it's going to be a killer...