If you just made that change inside Visual Studio with the IronPython Interactive open, you'll have to reset the interactive window (click the blue button on the IronPython 2.7 Interactive window's toolbar).
If you did reset the interactive, and it still isn't working, it would be interesting to see what the command line is that we're passing to create the IronPython process. In a Powershell window, do:
PS> ( Get-WmiObject Win32_Process -Filter "Name like '%ipy%'" ).CommandLine
On my system (where "import scipy" is working for me in the IronPython interactive window) I see this output:
<tt>"C:\Program Files (x86)\IronPython 2.7\ipy.exe"
-X:Frames "C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\Extensions\Microsoft\Python Tools for Visual Studio\1.0\visualstudio_py_repl.py" --port 9947 --enable-attach --execution_mode Standard</tt>