Also, will the new tuners be able to fit through the hole in the back of the sled? I heard the old ones could, but can the new ones?

The old ones could not fit through the hole in the back of the sled, either.

They might (depending on whether you've got a massive cavern inside your dash or not) have been able to go through the hole in the bottom or top of the sled. But not the back.

But I would not recommend trying to design an installation around being able to do this. Proper installation of the tuner module requires removal of the sled. In many car installations, this would involve properly unscrewing the console section or the dash section, perhaps even leaving the sled attached to it. Or maybe you've used an installation kit which allows the sled to attach to the kit and the kit itself can be unscrewed. De-installing the sled should not be a major issue for proper installation work.