I'm not sure I understand why efficiency would suffer. The starches should still convert just fine.
Also not so sure why you think color would be off. We're talking about grain, not old LME which will darken with age.
The flavor of the grains may not be as fresh, but I think that would be the only impact. I'm not even sure how that would translate through to the finished beer, but it could have a flavor impact for sure.
I say brew with it. If you're really worried, mix it in with some fresh grains to minimize the impact.