You may not feel any problems from low octane fuel & infact the owners manual says its ok to use on occasion (by accident) BUT you are probably hurting power. C&D did an article about 1 year ago with several cars using 91 & 87 octane in a dyno comparison. The car designed to run on 87 octane gained a slight amount of power on 91 (maybe 1-3, not enough to be worth the cost). But 91 octane rated cars lost more on 87, 5-6 HP or more. I can get 91 for 10 cents more per gallon (Amoco on Tuesday). A 5 HP gain would cost alot of money after cheap mods like filter & y-pipe. not "cost effective" to run 78 if power is a consideration. SVT Duratec is a special motor & deserves to be treated as such.
