Can ThreadSafe errors for ScriptingAdditions be eliminated without deleting the addition files?
As workaround, InjectEventHandler handler can be removed or commented out from Info.plist from the files:

/Library/ScriptingAdditions/SIMBL.osax/Contents/Info.plist
/Library/ScriptingAdditions/MySpeed.osax/Contents/Info.plist


Source:
https://code.google.com/p/simbl/issues/detail?id=7

In example:

--- a/Info.plist
+++ b/Info.plist
@@ -37,8 +37,6 @@
                &lt;key&gt;Context&lt;/key&gt;
                &lt;string&gt;Process&lt;/string&gt;
            &lt;/dict&gt;
-           &lt;key&gt;SIMeleop&lt;/key&gt;
-           &lt;string&gt;InjectEventHandler&lt;/string&gt;
        &lt;/dict&gt;
    &lt;/dict&gt;
 &lt;/dict&gt;


Or you can also try to set ThreadSafe to true:
E.g.

&lt;key&gt;OSAXHandlers&lt;/key&gt;
&lt;dict&gt;
    &lt;key&gt;Events&lt;/key&gt;
    &lt;dict&gt;
        &lt;key&gt;sysodlog&lt;/key&gt;
        &lt;dict&gt;
            &lt;key&gt;Handler&lt;/key&gt;
            &lt;string&gt;DisplayDialogEventHandler&lt;/string&gt;
            &lt;key&gt;ThreadSafe&lt;/key&gt;
            &lt;true/&gt;
            &lt;key&gt;Context&lt;/key&gt;
            &lt;string&gt;Process&lt;/string&gt;
        &lt;/dict&gt; 
...



  Handler dictionary keys:
  
  ThreadSafe - A boolean; is the handler
  thread-safe? (Event handlers only.)
  
  ThreadSafe OSA and AppleScript in Mac OS X v10.6 are thread-safe. If a
  script is executed on a background thread and invokes an event handler
  that is not thread-safe, it will be called on the main thread, which
  is slower than directly invoking it on the calling thread and may
  reduce application responsiveness to user input. Ideally, all handlers
  should be thread-safe. Some inherently cannot be; for example, almost
  anything that displays UI must be executed on the main thread. You are
  encouraged to either verify that your event handlers are thread-safe
  or update them to make them thread-safe.
  
  If the value of this key is true, then the handler may be executed on
  a non-main thread and it may be executed concurrently from multiple
  threads. If false, it will only be executed on the main thread. This
  key is required for event handlers, and must not be present for
  coercion handlers, which are required to be thread-safe. If a coercion
  handler cannot be made safe to run on background threads, the handler
  must arrange to execute the coercion on the main thread, such as using
  by using libdispatch and the main dispatch queue.


Read more:
http://developer.apple.com/library/mac/technotes/tn1164/_index.html#//apple_ref/doc/uid/DTS10003003-CH1-MAC_OS_X_V10_6__SNOW_LEOPARD__AND_LATER__INFO_PLIST

Related issues:

https://code.google.com/p/simbl/issues/detail?id=7
https://code.google.com/p/simbl/issues/detail?id=38

Other:

https://code.google.com/p/simbl/issues/detail?id=14
https://code.google.com/p/simbl/issues/detail?id=66
https://code.google.com/p/simbl/issues/detail?id=67
https://code.google.com/p/afloat/issues/detail?id=43