Refinacing student loan and extending term

I have finally decided that it is time to refinance my student loans. I anticipated paying them off in 10 years, I'm on my 5th year right now, but have decided that is foolish. Please give me advice on whether I am making a smarter move cash flow wise.

Currently pay 6.8% with a little over 5 years left.
Will pay 5.125% over 15 years. (so an additional 10 years)
Will free up approximately $450-525 in cash each month. I plan on investing $400 a month continuously in an account that will return me 8% in total investment in dividends over the course of a year. My plan is to build up the monthly return in dividends to pay the full amount of my loan and then never worry about paying it again. The extra $50-125 will go to savings and rainy day fund. Does this sound like a sound plan?