Here's the thing on exhaust. Knowing SCCA and rules and stuff, go as light as you want (or that you can, depending on state rules and sound and all that) for an exhaust. Then, on the hangars, and/or the exhaust joints, weld on some lead/steel weights, or just on any of the pipes to bring it up to the 18lbs min weight.
Don't let exhaust weights and stuff get you wrapped around the axles on which exhaust to buy. Go with common sense, keeping in mind noise restrictions for your event site.
--kC