"Well, without "knowing" what the 3rd driver is... does it measure well?"
i have no idea, since it hasn't been measured. a measurement will show things like 2nd and 3rd distortion and any strange frequency response behavior. good measurements will show things like spectral contamination, intermodulation distortion, and power compression behavior. none of that shows up in a winisd model. alternatively, you could do a klippel measurement on the driver in order to measure the parameters that are likely to correlate with the above items. the winisd model is simply to help you select the right size enclosure. it doesn't tell you much about the quality of the driver or how it will perform.
with a qe of .58, the driver has a pretty weak motor, which is why the f0 is relatively low at 21hz and that is good for getting extra extension in a an enclosure, but might not be ideal for other things. 180 watts rated power is pretty low. that is likely to experience thermal compression at just about any reasonable listening level.
i'm not really sure what the point of the "hide the subwoofer" game is all about, but i'll play along.