D:
You are correct in assuming that the chambered design of the WS will in fact be more restrictive than a straight thru desing like an Apex N1 or Comptech muffler. I'm not sure that I can recommend a straight thru muffler to you that will be reasonably quiet because I have no idea what your definition of quiet is.
The Hytech catback exhaust utilizes a twin pass system (similar to DC) which consists of two small straight thru mufflers to kill the sound. Apparantly its just a bit louder than the oem system. Other than that, most of the straigt thrus will be fairly "boomier" than a chambered design. As you know the exhaust from both of these companies are really pricey. on your application (LS w/ mild boltons) I think the whp compromise between a chambered WS vs. a straight thru would be fairly small, assuming that you sized the rest of your exhaust system properly.
Just an FYI....the Tanabe exhausts are not quiet by any means. I've heard the greddy evo (not the SP) and its about as loud as my comptech. PM Mike D or Surfer if you're interested in the SMSP or Hytech-I'm sure they can give you lots more information on the specs of each company's system than I can. Unless you go w/ one of those.....you're going to have to chose what is most important a quieter car or all out no holds barred performance.