When I looked at the kits, they seem to be nothing but some carpet, maybe a little plywood, and four trim retainers. You could save yourself some money and DIY.
I think you'll save about 30lb of the tail of the car. It will make a difference in your mileage and performance, but I suspect you won't be able to measure the improvement. Maybe you just want to do it because it looks good. I can see that.