What the title says.  Speaking about an IRS trying to put the power down in a straight line (drag race, etc).  I mean the grand total amount of negative camber relative to the road surface.  It's increasing as the body rears back and compresses the rear suspension. 

When does a (typical street radial) tire's straight-line grip really start suffering?  2 degrees?  4 degrees?  Does the problem steadily get worse as the camber steadily increases?  Or does it start worsening more rapidly beyond some tipping point?  

I guess I'm asking, how bad is bad for this issue?