I have also tried to do this, also deleted the hdd cache as well as the installed game and all save games but didn't reset, i have now deleted the game, saves and the elite soldier marketplace download and will install them all back on later have to go out right now, will let you know if it works.
Just so you know it did not work. I don't know what else to try. My only concern is that the game may differ in some way each playthrough, for example it may get harder or you may retrieve less weapon fragments, alien tech, affect achievements etc. If this is not the case then it doesn't bother me so much apart from the fact it looks like in now on my third play through when its actually my first proper play through. On a slight change of topic i have noticed no way to change the nationality of the soldiers, you can edit everything else this seems odd to me or am i missing something?