If one ate enough cake that would require him to bench (without intending to do so before starting ie without first washing and saying hamotzei) and then wants to eat bread would a new bracha be needed or since the bracha achrona is the same as if it were bread would the original mezonos cover it? would it be better to bentch and then have the bread or is that a bracha sheano tzricha?
If the person will eat bred after eating a significant amount of cake etc. he will still make hamotzei on the bread. Although the reason we bentch on a significant amount of cake is because it might be considered bread, nevertheless it is not a type of bread that one is koveah seuda on. Therefore, his bracha did not include actual bread, and he would make another bracha on it.
Sharei Habracha 16 ftnt. 94. Also see M:B 168-26, that if after eating a shiur kvias seuda, he decided to eat another shiur of cake etc., that he has to wash and bentch, since now he has a hamotzei food in front of him. If he makes hamotzei on that cake, he surely would make hamotzei on bread itself.