He's looking at the ULS 15, which is still sealed.
Like others have said, going to another pair of sealed 15" drivers is not going to be a real audio upgrade, just a visual upgrade, and a bit of space. For the record the Hsu's footprint is 18 x19.5, and the UM15's is 19.25 X 20. If you don't have any kind of DSP on the ultimax's though, you may gain some low end output with the Hsu's as they have some DSP.
I'd still be inclined to keep the current setup, get them painted or finished how you like, and add a mini-dsp if you want to get a bit more out of the ultimax's.