Actually you should be able to do so with a StyleScript:
<stylescript>
<remove>
<class class="SIB_StartPanelPriv"/>
<class class="SIB_StartPanelComposited::StartPanelPriv"/>
</remove>
</stylescript>
Save that as SIBClassRemove.xml in your WSB\Data\StyleScripts folder and then hold Shift and click Import Stylehack and select SIBClassRemove then click Apply StyleScript.