When I was looking into shorties I called and talked to a tech guy over at JBA about the very same thing. At one time they did make the 1 1/2 dia headers but they stopped making them a few years ago he told me due to low sales and the company didn't feel the need to stock the two part #'s for them. He was a really cool guy and told me most people didn't understand the whole exaust velocity/volume thing, but also the drop of in torque between the 2 isn't much at all but you could measure it on a dyno but you wouldn't feel it.
This is what I got from him and actually I was lucky he was around for the design of these headers. The 1 1/2 headers and 1 5/8 both have the firecone collectors which help with exhaust scavenging and increases low end torque, this is where you make the gain over other normal shorties when you can't go to longtubes.
1 1/2 headers are fine on a mild build. The 1 1/2 dia headers have increased flow velocity which helps with low end torque and low to mid range speed the 1 5/8 headers have in increased flow capacity and can push out more air but give up more low end torque and mid range output, moving the torque peak higher in rpm range. When they where designing the firecone the shape of the collector and some othe stuff if remember right either made this better or worse on the dyno, now all the headers have the firecone which improved the torque numbers and powerband.
This isn't a sales pitch and I don't work for them but I wanted to give you the best answer I could and this is what I got and is why I bought them, although i got 1 5/8(you can't get the 1 1/2 unless used anways). So I say go for it because either way from what he told me about them diameters when they did have them was it wasn't much of a differance flow wise, like a said the dyno picked it up but not enough to get to concerned about I think he said like 1-2hp.
