If you previously had no issue, i.e., you would see the ColdFusion error message and stack trace, not the generic IIS screen, and then all of the sudden the generic IIS 500 error started showing, my guess would be that your web.config got overwritten either by a global change or some other local change. I'm not sure how you had it set up before, but if you want to completely bypass the standard IIS 500 error msg and use your own built-in ColdFusion error (or simply see the ColdFusion "grey box") then you need to have this line <httpErrors existingResponse="PassThrough" /> somewhere in your web.config, like this:
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
<httpErrors existingResponse="PassThrough" />
</system.webServer>
</configuration>
There is a whole thread on this topic over here: http://forums.adobe.com/message/5388861
This goes for 404 as well as 500 handling. Not sure how IIS 8 differs from 7.5 but I would think these concepts are similar. Hopefully I'm understanding your problem correctly. I dealt with all this when switching from CF9 on IIS 6 to CF10 on IIS 7.5.