What is the happy medium between increasing 401K by 15% and adjusting W4 withholding?

[Critter-3]

The employer will automatically adjust your withholding if you increase/decrease the 401K contributions.  You don't need to hand in a new W-4 form at all.  

[Opus 17]

The calculations used by payroll companies (which are specified in IRS instructions publication 15-T, if you are curious) are self-adjusting.  If your taxable income goes down (because of increased 401k deferrals) then your tax withheld will also go down automatically.  If you are happy with your current year-end tax situation (size of refund or amount of tax owed), that should be the same without changing your W-4, because if your income increases or decreases, your withholding will increase or decrease automatically.