Why do you not just accelerate your current payments? This would in essence reduce your commitment term to 15 years or maybe less. There are multiple benefits to this as well. First you would have no charges associated with a refinance . Second you would not have to worry about the property appraising to meet a value that your lender would require. Thirdly and most importantly, if at some point in the future your financial situation would tighten, you would have the benefit of falling back on the lesser payment amount of the 30 year mortgage without any negative impact on your credit standing.
Keep in mind that extra payments that you make will reduce you principal amount owed, which will be of benefit to you later as your income to debt ratios will be enhanced.