Plausible - but not true.
Remember - when travelling between planets in ED, you're not in Einstein space, you're in some sort of bubble, probably mostly resembling an Alcubierre drive, with some other SciFi FTL features thrown in. What seems to happen is that your FTL drive in ED loses traction once the local gradient of gravity becomes too steep compared to your current speed - you don't speed up when diving into a gravity well, you just stop slowing down (or slow down slower): your tires are slipping, so to say.
In Einstein space, yes, you speed up when flying towards a gravity well, and you slow down correspondingly when flying out again - while in ED's hyperspace it's exactly the other way round: you speed up when coming out of a gravity well, and you slow down when going into one. The deeper you are in, the slower you go (try low waking from a 2g planet during a buckyball race when your target system is obscured

).
In an Einsteinian swing-by maneuver, you can gain or lose speed, depending on your direction of travel compared to the planet's direction of travel (basically, the planet drags you along while you're close to it) and whether and how you fire your engines - but the speeds we're talking about here are decidedly non-relativistic.