From what I can gather, the actual surface area of the topper is not much smaller than the Firm Fanny Lifter. Therefore, I think using the step risers and the topper in place of the FL is a perfectly acceptable option. I am going to buy a high step because I imagine that this will be more stable than my FL, which can seem wobbly, especially on one legged squats where one foot is on the box. I also like how you could adjust the high step for different exercises. While I like the height of the FL for step ups (it's perfect for me), the 14 inches is a bit low for sit and stand squats and my knees really feel it. So for that I could add 2-4 inches and it would be perfect.
IMO,I think the high step with risers offers more stability and more options for height than the FL.