ColdFusion 10 Update 11 404 handler not firing

Question

I know CF 10 has a number of issues surrounding 404 handling. This seems to be different from the other reports. Details:

• Win2k8 R2/64 and IIS7.5
• Upgrading from identical config on separate server. Only difference is CF9 -> CF 10. All works fine on CF9. Adobe CF9 Lockdown implemented on original server, CF10 Lockdown implemented on this server.
• missing template handler set in CF Admin as /404.cfm, which should translate to the Cfusion root (c:\ColdFusion10\cfusion\wwwroot).
• IIS has been config'd to trace failed 404 requests
• IIS 404 handling is default (originally executed a CF URL but removed to simplify debug).
• Coldfusion webroot where missing template handler resides is default install location
• IIS site root is entirely different: c:\Other\Place\SiteRoot\
• A sitewide error handler is also set in CF Admin in the same ColdFusion webroot and works as expected.

404.cfm is very simple:

<cfheader
   statuscode="404"
   statustext="Not Found">
<h1>404</h1><p>Page not found</p>
<cfoutput>#now()#</cfoutput>

Inputting the bad url [domain]/foo.cfm should display the above template. Instead I get an IIS error screen. The CF missing template handler is ignored. The IIS failed request trace says the url is

http://[mydomain]:80/jakarta/isapi_redirect.dll

and a detail view shows at Step 1

RequestURL="http://[mydomain]:80/foo.cfm

I've seen plenty of issues surrounding CF10 and 404's, but never that the missing template handler assignment is completely ignored. In CF9 this will generate output as expected. Anyone seen anything like this?

EDIT:
I have also tried config'ing this to match a different CF9 server I have running: Added a CF mapping to the web root of the site. Then placed the missing template handler in the web site's root rather than the CF default web root, lastly in cfadmin pointed to the missing template handler in the web site's root using the mapped folder. Same problem. Works fine in CF9 and not at all using CF10.

EDIT2:
As Miguel F pointed out in the comments, you can shut off HTML error codes in CF Admin and this will let the Missing Template Handler fire... BUT you get a 200 header to go with it. Apparently cfheader statements are ignored as I have tried placing the cfheader at the beginning and end of the template... still yields a 200. Visually fine but insofar as SEO is concerned thats a disaster. Just looked and my CF9 servers do not require this setting to be unchecked for their handlers to work.

EDIT3
Dana Kowalski's solution displays detailed IIS errors to the public, so for a 404 on, say, a made-up extension (foo.xyz), the screen will show file paths. Default behavior is to NOT display detailed errors except when running templates locally, and display custom error pages to visitors. The CF error template should work fine with that setting.

POSSIBLE SOLUTION
I stepped back to ColdFusion 9 as part of debugging this problem, and may have discovered the solution while debugging a separate related issue.

Answer 1

In the IIS Manager, click on the site in question. Select the Error Pages option. Select the 'Edit Feature Settings' link on the right side. Check that the 'Detailed Errors' option is selected.

If you have either of the other two selected, there are times IIS 7.x will take over and not let ColdFusion handle it.