Well, the numbers that Josh got from the driver were in a 4cuft box, and the one I modeled is about 2.5cuft. I don't expect the numbers to be too far off from what he got, but it'll take a lot of power. You have to remember the the SI drivers weren't made with ULF in mind. They were designed to give their best performance above 20Hz. The HPF, for now, is to protect the amp and even with 200W you should be able to get close to 100db or more between 10-20hz and ~110db between 40-80hz*. If you do choose to add more power, you'll still want to add a HPF somewhere down there 10-15hz, just to protect the driver from reaching xmax.
*ignoring room gain and doubling up.