Thanks everyone! To those who are saying it will go to zero (or something close to it), and I'll basically lose everything, I realize that if I were to put 100% of my money in it. But in my first post I specified that it would be something less than that, like 20% or 25% or something.
So, let's say I put 20% in the TQQQ. Then 80% call it in a REIT fund (so the TQQQ is the risky stuff, the REIT fund is the safer stuff, probably a bit less volatility than the average non-REIT stock). That would put me at (effectively) about 60% in QQQs, and still 80% in the REIT, so I would still only be 100% invested, but would functionally own about 140% of my portfolio, but no margin interest. 140% over the long haul would almost certainly kick the shit out of any regular portfolio you could come up with.
And, for that period where TQQQ goes to (almost) zero, since I only have 20% of my account in the TQQQs, my account is only down 20% on that position (and whatever my REITS would drop in value as well of course). So the account would of course would be more volatile, but not a risk that your account would go to zero (or close to it).
But it gets even better (I think). Since this is in my 401k, I would be putting money in over time (as amounts are deposited from my work pay). Given the huge volatility of the TQQQs, the DCA would almost certainly pump the returns considerably (the more the volatility of the underlying, the more DCA generally benefits).
So I think this could be HUGE.
I need to run some testing on this.