Hi Craig, I think the answer to your question is no. I don't have experience with that particular card but my following comment is based on this information.
http://www.sonnettech.com/product/crescendo_nubus.html which indicates it is a PDS slot card.
The 7100 used the PDS slot for optional video cards and not a nubus slot so unless the IIci has a PDS slot compatible with the 7100's I would say no. Apple did ship higher performance nubus video cards for nubus Macs and the aftermarket also supplied them.
The Apple card I was thinking of was this one, the 8•24.
http://lowendmac.com/video/apple8-24.shtml and there is also a page of links here for other nubus video cards,
http://lowendmac.com/video/index.html
I seem to recall we used the Thunder 24 cards in the Quadras at work which means I still have a couple stuck in the machines we EOF'd back then....
Kevin