OK - I tormented my self googleing to find out why SH404SEF was not correcting my J15 sef urls. Couldn't find a darn thing about it. So, I turned to the code. Sure enough, it does do J! Sef, but only 1.0 - NOT 1.5 sef.
Here, I'm going to make some notes on how I view SH404SEF could fix 1.5 URLs.
administrator/components/com_sh404sef/sh404sef.class.php - Line 3562
We know J15 Sef builds URLs like this: (blah)/(id)-(alias) - Therefor, if we search the content DB for those items, and they exist, return "content15" (content15 to distiguish it from 1.0)
Ignore the blah -this is part of the menu system
components/com_sh404sef/sh404sef.inc.php - Line 206
Need to add content15 to this, since it's now a valid J Sef.
Same file - After shDoSecurityChecks call
Need to actually figure out what it would be redirected too and do the redirect - maybe to a com_content url so it allows for a new auto-redirect
Need to also watch out for sef_ext.php - seems to be called in the sh404sef.in.php file...