They will probably be just as productive as all the other employees but they will be sightly unhappy about how incredibly lacking the non-google tooling is because they know how productive someone could be.

Source: Googler but still writing lots of code outside of Google and constantly think to myself "why is this so needlessly hard? Why does nothing integrate well?"

That translates to "They'll be as good as others, but far more likely to leave" doesn't it? Still not a good signal for hiring.

I'd rather say that they are far more likely to help improve your tooling and productivity because they have been exposed to Google tooling and know what development _could_ be like. Even if they just do incremental improvements that others would not consider.