From what I have heard others say cobra irs is junk and a solid axle with ta or 3 link will preform better. Mods in sig but I have been looking into doing a irs swap just wondering pros and cons of both.
On a 86 Coupe, I think a true 3-Link live axle set-up would be the route to go on a weekend track car that's still streetable.
I hear good things about the Evolution Motorsports set-up as well as the MM TA
http://www.evolutionmsport.com/evmCart/index.php?main_page=product_info&cPath=1&products_id=2
I wouldn't say the IRS is junk, just a lot of compromises. ;-)
AI and AIX cars have gotten the stick axle to work pretty good.
Most of the people in this forum do drive their cars on the street in which a solid set-up as you describe above would not do well on the street.