I've done lots of 300k-400ks on GP4000/5000s, 25 or 28mm. Like them better than the Compass/GKs at that width, but all of them were fine. Have also done one mixed-terrain 200k on 38mm GK slicks (different bike) which were just fine too.
But, I will say: 14mph vs 16mph is a big difference. That's two hours on a 400k. (And two hours of SLEEP on a 600k, but it doesn't sound like you care about that -- yet.) Though I'm guessing slick vs non-slick is a lot of that, rather than any other differences in the tires.