Some people allready have a business in FC conversions. On the bottom of this page //www.ch-audio.com/CH288287.html there is a measurement comparison between a stock alnico and a fieldcoil converted 288 driver. Clearly the field coil is smoother and more extended, surprisingly (or not?) most at the bottom of the FR. I would expect more difference at the top end if the differences where only due to the FC having a stronger field.

As least as interesting is that the driver has a very straight FR from 150-15kHz! Very nice for any full range driver. Now you just only need a suitable horn, like an WE-15A .