If they figured out how to make female space marines, would the Imperium gain any significant advantage from essentially doubling the amount of space marines they can produce?
Well... yeah duh they would. Having twice as many space marines is only a bad thing for the enemies of the imperium at this point. Though it wouldn't be enough to make a significant difference on the galactic scale and isn't gonna happen since the gene seed they use to make space marines is incompatible with females because the Emperor distilled them from the genes of the Primarchs, which were all male. If there were a female Primarch, maybe. But there isn't so no.
As a reminder, 40k science is founded on bullshit and machismo. Biologically that's actually bullshit.(one chromosome, oh no so hard to change as the Emperor of Mankind).
If I remember right and the source was accurate, he did it on purpose to prevent possibility of Space Marines becoming self-sustaining race of tyrant kings. Although he could have just made the females barren, but I think it's fairly well established he was at least somewhat sexist and didn't always make the best decisions, as indicated by the Council of Nikaea and, you know, the whole Horus Heresy thing.
Also as far as I knew the Nids avoided Necron worlds because the biomass cost to biomass reward ratio simply wasn't worth it. I mean, Necrons controlled one galaxy? And corrupted the Warp in this one location? Whereas Tyranids have, in all likelihood eaten several galaxies, and probably found a way to eat the entire planet rather than merely the juicy bits, along the way? Unless Necrons all spontaneously awaken, unite their fractured dynasties, and put down or quell the Imperium enough to get shit done, or Chaos wins and everything is daemons, or Orks win just in time for the WAAAGH the size of a galaxy to turn on the Tyranids, the galaxy is fucked. Or, since we're talking about such impossible scenarios(by which I mean GW advancing the plot line or changing the status quo), we can also say that if everyone came together in peace and friendship and allied all of their might against the Tyranids by making use of all of their best technologies freely shared and free of suspicion or religious fanaticism,then they might have a chance. Hell, at that point, they've already accomplished the harder of the two tasks.
That said, fully awakened Necrons would be pretty damn scary.
But so would a Tau Empire if it was about 20x it's current size.