I've only flown with Kaia in cabin and I've gone one way not sedated (which was really horrifying) and then I gave her half of the recommended dosage the other way. Unfortunately, Kaia freaked so much off the anxiety medication, she vomited a few times and shook. It was a really long flight so I figured it'd be best for her to sleep. I didn't have the best flight conditions either, both during icky weather and 5 hours on bumpy flights even gets me super anxious.
I have also heard horrible things about cargo'ing a pet. With a small sized dog, I'd *personally* consider booking travel with a carrier that only allows pets in cabin. If you can't change your flight (I know most flights charge for cancellation.) I would read this:
Ask George: Tips For Pets Flying in Cargo? | TravelPost Blog
It says from the blog about sedation: 1. Do not tranquilize your animal. The biggest danger for pets traveling by air, especially in the cargo hold, is dehydration. Tranquilizers dehydrate animals even further, making it especially dangerous to medicate them for air travel. Many airlines will not even take a pet they suspect has been tranquilized. Yes, the experience of air travel can be traumatic, but your animal will get over it. Better not to take the more grave risk to its health.
Best of luck and post some pictures when you get a chance! I'm quite envious that you get to spend a month in Greece, be sure to update us on the adventure =)