+boxfish+
Don't overlook the possibility that you're swimming normally, and that your buddies need to adjust their kicks. There is nothing worse than diving with someone who kicks endlessly and at a rapid pace. Diving is not racing. Ask your buddies to slow down.
Also, you might want to switch to an alternate kick, allowing your muscles to rest. Alternate between your regular flutter/scisor kicks with frog kicks. Remember that the frog kick has a glide component. Don't kick again until you stop gliding. Pay attention to your trim as you move through the water. A horizontal position reduces drag and improves the efficiency of your forward motion.
Just food for thought before you decide to purchase another set of fins. The Mares Avanti Quattros are really nice fins. I used them for many years. My buddy's wife is about 5'1 and hates the gym. She uses the Scubapro Seawing Nova and is never tired.