My understanding, which is pretty much yours and Silverlady's, is that the sural nerve biopsy would reveal the aspects of the disease that make up CIDP, which is primarily a clinical diagnosis. That is, it would reveal that the neuropathy is primarily demyelinating--usually, that is revealed through examining the remaining myelin for damage from immune complexes and cell infiltrates, resulting in inflammation (and, of course, finding a lack of myelin, or damaged myelin, is also a big tip off)--and for evidence of a relapsing/remitting pattern, meaning attempts at remyelination (this often leads a characteristic myelin pattern known as "onion bulbing" around the cells).

On the other hand, the sural nerve biopsy itself is not likely to reveal the original cause of the CIDP, just the syndrome/process. Specific antibodies are unlikely to be identified this way (and are just as likely to show up in blood testing--which is to say, not often).

A sural nerve biopsy has much higher yield in autoimmune neuropathies that are primarily axonal in nature. This are more likely to be asymmetric and to have vasculitic components--often, these involve variants of the anti-nuclear antibody.

A sural nerve biopsy wouldn't seem indicated for Alan, as it wouldn't alter his treatment. And yes, while there are some who've had sural nerve biopsies and come through with no permanent damage, it's a tricky procedure, best performed by a well-trained surgeon--there are plenty of people who've experienced permanent pain or numbness in the area afterwards.

See:

http://www.neuro.wustl.edu/neuromusc...mdem.html#cidp

http://www.neuro.wustl.edu/neuromusc...dy/pnimax.html

And the section Silverlady listed from this site is also a good explanation of the circumstances under which a nerve biopsy would be indicated.