re: When does it make sense, mathematically, to accelerate mortgage payments?

I wouldn't think about early payoffs on mortgage less than 3%. That's gold right now.

If you aren't disciplined enough to invest the monies elsewhere now, what makes you think you will be disciplined enough to do anything productive when you stop paying your mortgage payment a month. And people forget, really, you're only not paying the P&I once you pay loan off. You still should be saving for taxes and insurance (don't get me started on savings not paying).