You don't need a tune for just a CBE. Once you do the downpipe you have to get a tune. With just a CBE, you can still tune but it's not required.
Downpipe = COBB Stage II Tune (or custom protune)
CBE= No tune or COBB Stage I Tune (or custom protune)
If you're planning on getting the ap you can just flash the car to stage I whenever you want, then go stage II if you do the downpipe. If you go the custom protune option, it'll be more cost effective to wait until you get the downpipe and just get it tuned once.