I have a duplex that is basically breaking even but we will save $550 by going from a 5.5% fixed down to 3.5% on the remaining principal (no cash out).

I reckon the "principal-only" repayments (rather than the more usual "interest-only" repayments variation) is the BIGGEST plus for this deal!

I can refer you to my investing group's financial experts (accounting firm / law practice), if you like.Where the issue may be getting confused is that the interest on financing is an operating expense for income property, but the principal portion is booked under debt service, as I understand it.

You can refi up to the HML principal (+ closing fees?)

If you do sell within the next three years, you'll probably come out even, as you added the additional loan costs to the principal you need to pay down and your house will may be underwater (assuming you bought at market and there isn't crazy appreciation where you bought your house). 

I am a principal commercial broker and also a commercial developer and I have plenty of commercial opportunities where I live.I do not invest in SFR.

Would you and should you somehow use that cash to pay down principal first and use your own salary to pay monthly mortgage payment? 

While principal paydown isn't usually going to come in to play in a 5 year period, you will also have more equity from the very beginning due to the value add.

If that number factors in maintenance, vacancy, capex reserves in addition to both principal/interest payments and the taxes and insurance I would definitely do the deal. $2580 of annual returns on a $0 investment is a hell of a return. 

When you know what someone wants and giving them what they want gets you what you want principals are a foolish waste of energy.