Thanks again, everyone. I didn't mean to abandon, just haven't had the time to check back in. Radagast, really appreciate the breakdown.
Lots of good inputs. The idea to just invest the money and see where I'm at is an interesting one. I would have paid out about $57k in interest in 4 years under normal payments, and if I'm understanding things correctly, it's still annualized to about 4% in interest (even though it feels like it should be more) when considering the full $376k I'm putting away.
I did a couple scenarios. 1) If I stick to my normal mortgage and invest 1k/mo + 20k/quarter @ 7.5% returns (plus equity minus interest). 2) paying off the mortgage with $23k/quarter (limitations of this amortization calculator), so plus paid-off principal minus interest. Former was about $402k (after 15% capital gains tax), latter ~$351k. Realistically the latter is a bit better since the interest is deductible, and the former is also probably better as I won't immediately pull the money out. I know over the long term, investing _will_ come out ahead.
If that sounds like reasonable calculations, then then I think the opportunity cost of paying off the mortgage is small enough to not warrant paying it off. I'm starting to lean towards YoungGranny's advice of investing for a few years and maybe making the decision to sell and pay off.
Though I do agree with "Bird In Hand" as it pertains to my situation, because between large fallback funds, job security, and plenty of opportunity, even if the market tanks and my house is gone, I have a high likelihood of surviving.
If I was doing it again, I'd put the additional principle into closed end municipal bond funds purchased at a discount to NAV.
I don't know what this means, but I'm interested.
If you could edit the thread title to say 'instead of after-tax investing' that would make this topic somewhat less controversial.
Good call! I've done just that.