My wife also has a 2008 GS.  She has ridden in parades, stop & go traffic, etc. during 100 degree Texas summers and never had an overheating problem.  I think you have something going on.  Maybe low coolant, old coolant, empty/cracked reservoir?  I can't say what the problem might be, but, although they are helpful, you shouldn't have to add aftermarket parts to fix the problem.  Good luck and be safe.