I understand your question, and unfortunately, there's no easy answer to this one. It really depends on what you want in the future, and where you are right now. Without knowing your cash flow I can only offer generalities.
Contributing less than the 5% that is matched would mean you are giving up free money, so I'm glad that you recognized that and will at least continue that.
Pre-paying the loan will do nothing towards making your retirement grow, but will pay off in the immediate future by eliminating the debt, which as you pointed out has no tax deductions
If you can easily afford the 15%, then you're still making your retirement funds grow for the future.
So the answer to your question depends on your cash flow really. If it is a hardship to pay the loan AND contribute 15% to your plan, then bring the contributions down to the 5% they match and pay off the loan.
If you can easily afford the 15%, then continue that rate
of contribution. This also takes that contribution out of your taxable income
since this is a pre tax program, which is another variable in this scenario.
This is as much a personal
choice as it is directed by cash flow. You can also change any variable until you find the one that gives you the best option right now, such as contributing 10% instead of 15% and pre paying the loan if that works better right now.
I suggest you sit down with either a calculator or a financial adviser and run the numbers to see what works best for you.
I hope this helps.