I've had them within 5 steps with no foliage on the trees..
I've many times had turkeys within 5 steps while I was wearing an orange vest & orange cap.
I'm sure ghillie suits help a bit with camouflage, but turkeys are much less likely to be alarmed by a stationary human than a deer. It is mainly movement (as well as unnatural sounds, like velcro & metallic) that spook turkeys.
I'm sure there are situations where these ghillie suits are more asset than liability, so not criticizing anyone for using them. I was just more focused on how they hung up on briars, and didn't like wearing them while turkey hunting. For me, they make more sense for deer & coyote hunting.