I work for my family's construction business. I've been trying to talk them into doing a NQDC (non-qualified deferred compensation plan) like you mentioned. There was another thread a few months ago on this topic.
If you are THAT sure about your employer not going bankrupt - all assets in the NQDC plans are paid out to creditors in bankruptcy before ever going to any employees that have a claim for the NQDC - then it can definitely make sense.
Your Medicare/SS taxes are withheld from the contributions, so you won't be getting a full 80% salary deferral. But close enough.
Also, check the investment options available. They can vary widely, even more widely than your 401k options, and possibly be even worse, since the company will most likely have different investment offerings in their NQDC plans vs their 401k plans, and possibly even have different administrators for it. While it can save on taxes, if your investment options are truly pitiful, it could very well make sense to still pay your income taxes now and be able to have full control over the investments. Over 10 years, if your investment options in the NQDC plan are crappy enough, you could very well be looking at a 2% differential in total expenses/returns between what you could invest it in in a taxable account vs what the NQDC is offering, and over 10 years, that could add up.
Also, when the money is withdrawn from the NQDC, it's taxed at ordinary income rates. So any capital gains or qualified dividends earned in that NQDC account will be taxed at your full income tax rate, not the 0%-15% long term cap gains/qualified income rate if you had taken it as income and invested it on your own.
So, if you have really crappy investment options, and your gains you withdraw are taxed at higher income tax rates, it could very well be almost a wash.
You'd also want to look at what your income tax rates would be now vs in the 10 years you are retired, both federal as well as state.
One thing that's confusing me is your %ages you reference. You say 20% will max out your 401k, which suggests a salary of about $100k/year. You then say in 3-4 years you can defer 80% of your salary and arrive at $1.5M in pre-tax assets, and then spread out the distributions over 10 years with $150,000/year.
Where are you getting $1.5M from in 3-4 years, when your salary is only around $100k? Do you have massive bonuses each year? If you have existing 401k assets, you cannot transfer them into the NQDC plan and take them out over 10 years with the 80% voluntary deferral. Your 401k has to follow the rules of nothing before age 59 1/2 (or 55 if you retire/are laid off), unless you do an SEPP. It's a completely separate deal from NQDC plans.
[edited to add: I see where you mention the 20% will be 401k as well as living expenses, 529 plans, etc. - so it's obvious the salary is a bit more]