I had the Nashbar ones on my Xtracycle and loved them. The easiest and cheapest thing to do is if you are set on buying them, buy them. Use them with your current stem and see how they feel to you. If you feel after you have used them you need a longer stem, then you have an idea of how much longer a new stem should be. I had a 12 mile round trip commute and the way in was on a flat open road, headwind the whole way. I would just use the bars themselves like aero bars and hold on to the top and duck down. My normal position was on the sides, kinda like the hoods of a drop bar. That was really comfortable.