The only reason you are lowering your interest payments is because of the lower interest rate. If the rates were the same, a 30 year mortgage vs a heloc, the interest payment would be exactly the same. Of course you could pay more on a 30 year mortgage and that would cut the amortization schedule, if you double a 30 year mortgage payment, it will be paid off in less than 10 years.
So once again, your interest payment is only less because you got a lower rate. But of course you paid a fee for it, but in your case over 2 years you would save around 20k in interest. After that who knows what will happen, the rate will go to 4.5% but you are not even sure of that because it is variable.
The whole purpose of the fixed 30 year mortgage for investors is because the rates are FIXED, and the payment is the same throughout the mortgage. So, no need to run and cry when rates shoot up. Also, your cashflow is pretty much set for the life of the mortgage, actually rents always go up, unless the neighborhood goes to crap, so you know exactly what you are getting.
You had a mortgage at a pretty good rate, now that you paid it off, when your 1.75% intro rate expires, you will need to try to refinance that money again. You took a risk, you are betting that interest rates will remain low and that you will still be able to refinance at that time. The bank is betting that rates will go up and you wont be able to refinance, you will find out who won the bet in 2 years when your intro rate expires...