The prepayment decision and the refinance decision should be made separately, even though they are connected.
Prepayment is an investment decision.
Whether you are aware of it, you have decided that the extra money you have available each month would be better invested this way than in stocks, bonds, real estate or collectibles. Eight percent is a pretty good return on a riskless investment. Because you are not looking to raise cash, your refinancing is a cost-minimizing decision. You must determine whether shifting to the 15-year loan at a significantly lower rate will generate savings in excess of the cost of refinancing. You answer this question first, recognizing that the answer is affected by your prepayments.