It is important to note that the printed values on the card are what the Ingester looks at when calculating the increase to its own power and toughness. If a Runeclaw Bear were to have its power and toughness increased to 5/5 by Giant Growth and then exiled by a Phyrexian Ingester, the Ingester will only get +2/+2.
One other fact to keep in mind, is that some creature cards have variable power and toughness. The initial characteristics of these cards are set by characteristic defining abilities. An example of a card like this would be Darksteel Juggernaut. These characteristics are set regardless of what sone the Juggernaut is in. This means that the power and toughness of a Juggernaut in a graveyard will be 4/4, if the owner of this card controls four artifacts. This also applies to the Ingester. The bonus granted if a Juggernaut is exiled will vary.