Investing/Strategies / Retirement Investing

I am trying to decide if I should bump up my 401(k)
contribution (beyond the minimum required to get the full
employer match). And I can't figure out how to account for
taxes.

Assume a marginal tax rate of 31%, combined state &
federal.
In my 401(k), the only real option is an S&P500 index
fund. Outside the 401(k), I can use higher growth
strategies, such as Foolish Four and Keystone Growth.
Assume a ten-year period, with these average annual gains:
S&P = 12%, FF = 16%, Keystone = 20%.

Suppose I have $1000 to invest.
Outside the 401(k), with FF:
$310 goes to tax, leaving $690 to invest.  Gain of 16%
reduced by 31% tax = 11.04% after-tax. In ten years, $690
compounded at 11.04% grows to $1966

Outside the 401(k), with Keystone:
$310 goes to tax, leaving $690 to invest.  Gain of 20% 
reduced by 31% tax = 13.8% after-tax In ten years, $690 
compounded at 13.8% grows to $2513

Inside the 401(k), with S&P: $1000 compounded at 12% grows
to $3100. Reduced by 31% tax = $2139.

But how do I treat the immediate $310 "tax savings"?  I
invest $1000 but my take-home pay is only reduced by $690.
Is it already fully accounted for, or not?  
The more I think about it, the more confused I get.