I big block of Sarus equiped with spears would definitely be vulernable to a flank charge. The example the comes to mind is Chaos Chosen charging the flank while the anvil like Chaos Warriors HW+SH (2+ sav) charge the front. Watch your spear wielding cold blooded friends melt off the table. A big unit of spear wielders, despite its damag/round potential is also going to have a pretty high SCR which getting the GW unit in the flank gives the opportunity for extra casualties and the removal of any rank bonus.
Perhaps chosen is a bad example of a flank charge considering that I would never take a flank charge by infantry unless there was no other option.
We were discussing these types of units at our last game session and like you mentioned at the end of your post, spear sarus are nearly as dangerous on the charge as when they take one. Provided the GW unit sticks for a round or two, the spike of initial casualties will be followed by a doubled attack count in the following round. In conclusion, taking a charge from GW's seems advantageous to the spear sarus, despite high casualties in the first round. Charging the GW unit (provided you can pull a good one off) spells almost certain doom for them and you could experience a crushing victory if you roll well in the first round eliminating strike back possibilities. If for what ever reason, the GWs hang around, you'll be pounding 20+ attacks into them before they have a chance to respond in the opponents turn. So charging that unit sounds like a great idea.