I beg to differ. It depends entirely on what was modified. Refinishing the handgun does far more damage to the value of this gun then simply modifying the cylinder. Also permanent modifications to the original sights would also far effect the value than reaming and counter boring the cylinders.

Cylinder modification was common practice on 455 handguns in general, whether that be a Webley Mark VI or a Colt New Service 455. It was such a popular modification that Smith and Wesson includes that reference in factory letters of the 2nd model 455 hand ejectors.

Saying that cylinder modifications have little value is just not accurate. A Webley Mark VI in its original condition goes for around 900-1000 while a shaved 45ACP Webley goes for around 600. The market in general would certainly challenge your assumptions of "little value". Value, as with most things, is in the eye of the beholder.