If we win the FA cup, yes its enough. In fact I think winning the Carling Cup is enough at this stage. Its not the destination we want, but its a sign imo of progress.
I get what you mean about feast now but fasting later, but I'm not sure it applies. In some ways I think that would have applied if we'd gone out and spend 100M on proven players rather than on younger players who are likely going to be better in 3 years than they are now. The first path may have helped us get into the top 4, but unless we kept spending like that every year, (like City) I doubt it would have resulted in a title run.
But our route I think or hope will have wet the appetite. Taught younger players what winning is about, and get them hungry for more success. Our problem this term imo is not that we have a bad set of players, but an inconsistent set of players. Bad players are consistently bad. We've already proven our quality already with a tough cup run, and may prove it further if the FA cup continues. The trick now is to take players with quality, add to them, and with experience make them more consistent.
Besides, to take your analogy one step further we could take a look at Arsenal who've maintained, and look to again maintain a top 4 finish. They've not had a feast for a while, and are starving in the process. If we consider the CC as nice snack, the FA cup as a nice starter, it may take awhile, but I'm sure the lads will be looking for the main course soon.