[fastd]

I'm no expert but...

figure out what is making your rear end loose...bad design, improper installation or otherwise.

Jury is out as to whether you'll pick up performance by switching to an IRS (sacrilege I know). Ride height adjustability clearly better with the IRS, other than that, it seems to depend about quality of parts and installation.

Figure it costs at a minimum, what $5k for an IRS? versus what is the cost of fixing up what you have...

Might help with value if you were to sell your car...i think it depends on what the total package looks like to see if your car screams for an IRS or whether buyers just wouldn't care.