Have done extensive tests and restarts with the Experiment, can confirm that Body Parts only give stats when they come directly from the corresponding Mutation. So you only get the speed buff if the wings are from the Mutation, but if your Experiment starts with only the wings Body Part, you'll still be able to fly and it doesn't take up your back slot.

Experiments might need some reworking. For example, some mutations that they can start with don't overlap, like claws/tendrils, horns/antennae and  wings/dorsal fin, so they're just wasting mutation rolls. Might also be better for them to not get extra Body Parts at the start, and instead only gain them through extra starting Mutations, just to keep things more consistent.

I haven't had it happen yet, but I wonder what happens if you've got the Wings Mutation, and an enemy cuts off the wings, do you still retain the speed buff, since you've still got the mutation itself?