Review response to quite positive review comments - first grant
I assume your question is about one of the UK Research Councils, probably EPSRC. If that's true, then 5555 is currently considered quite a low score, and such a proposal is not likely to be put forward for funding during the Panel meeting (unless you have a year with unusually low competition level, or someone in Panel is really keen to fund your research). In fact, the competition levels in EPSRC right now are so insane, that even proposals scored 6666 are not always supported. 

The fact that you have "lazy" comments from Reviewers makes your situation even more difficult, as you are not given a good chance to stand for yourself by answering the questions. Your mentor is right that at this stage you have to write a really brilliant Response to Reviewers to make your proposal fundable. 
Good luck with it.