I was going to work out what it would look like to pay down the mortgage faster vs investing...but it's too much work for the moment
Basically, paying down the mortgage is 100% guaranteed investment for whatever the interest rate is. Investing vs paying down means that you do get to save some additional money each year in terms of the mortgage interest as a tax deduction), but the results of investing carry the risk associated with investing. So, say you put 1k/month towards the mortgage, you're paid off in just over 9yrs...and then you have 2k/month to put towards investing...vs the 9yr head start of investing 1k/month minus the mortgage interest of those 16yrs (subtracting tax benefits, though).
So I did some simple calcs.
1) You don't pay down the mortage. So, from ages 25-50 you invest 1k/month tax sheltered (to make simple). Then 50-65 you invest 2k. I have not included the annual tax rebate of 2k...though that would change things a little for the first 25yrs. But, a simple side calc and I'll add it to the total at the end. Thinking out loud here.
2) You pay 1k/month extra to mortgage for 9yrs, then you invest 2k/month till 65.
Both assume 8%/year return. Both reinvest the annual 2k tax return (1. for 25yrs, 2. for 9yrs)...gross oversimplification I know but whatever.
Scenario 1 earns ~4.1mil on fewer deposits than scenario 2.
Scenario 2 earns ~3.1mil.
So, you're looing at a 33% increase in portfolio value from Scenario 2 to 1...so, factor into risk/variance, and make your decision.
It's not a clear-cut rational decision because you have to identify personally risk tolerance and that's not a black/white issue at all. There is no one correct answer (this is just a fact, not namby pamby comment). Personally, I'll take the paying down the mortgage. It's less risky AND the extra return of taking the risk is not sufficient for me.