It should be fine, the lpx are corsair's low profile vengeance ram sticks. They're about as short as you can go for ram. The only other thing to consider is the offset of the heatpipes angled (usually) toward the back of the case. X99 uses ram slots on either side of the cpu and the heat pipes may interfere with the left hand ram slot closest to the cpu.
Here's an example looking top-down on an x99 board. The ram slots to the right of the image are the slots between the cpu and rear of the case. I've highlighted the angled heatpipes in red and the yellow line represent where a low profile stick of ram would fit in that first slot behind the cooler, showing the potential conflict where the red and yellow guide lines overlap. There's a...