No clue. But, I will bump the post.
SSA taxes are calculated on the gross not on the net after insurance and 401k. Take a look at last year’s W2 and you will see that SSA wages in box 3 is higher than wages in box 1. Income tax is calculated on box 1 but FICA is calculated on box 3.
Here is a sample W2 to see what I am talking about.
http://www.irs.gov/pub/irs-pdf/fw2.pdf
Section 125 (sound familiar?) of the Internal Revenue Code (Ronald Reagan 1986) allows an employer to offer benefits to employees which are exempt from FICA and Federal income tax. For instance if you make $1000 per week but your employer charges you $200 for your benefits it’s as if you only ever made $800 for tax purposes! As long as you signed a form saying you’re happy to do this.
Now if you wanted to go back on this agreement years later I’m sure they would look at you sideways and be happy to charge you ALL the back taxes they could find, with interest.
Perhaps the solution is to calculate your tax savings and invest the difference. That would be the prudent and conservative solution IMHO