Shops who offer other aplications than the Mustang II often point out that they do "NOT use Mustang II geometry".
Is something wrong with that setup?

Have a look at Jim Meyer Racing Products:
http://www.jimmeyerracing.com/ifs5.html
They offer IFS setups based on GM mid size cars (with rebuildt spindles to make the Omni Rack rear mounted)

Is this setup better than a Mustang II setup?

When I say "Better" I mean geometry and handling.