One disadvantage might be that making the deck higher towards the sheer may reduce your ability to have an efficient stroke close to the centreline of the kayak.

The further out you paddle is, the greater the proportion of the paddle stroke that has a turning effect as opposed to propelling you forward.

However, the effect will be quite small and will be even less significant if you are quite tall, if you already have an inefficient technique and/or are not concerned about maximising the speed or distance travelled for the amount of effort you put in.

It might be a problem if you are short and have a strong competitive streak. :)