Image size above 1536px for iBookstore?
The main disadvantage is the current iPad will not be able to show the whole image with the high resolution as it was intended to be seen. Unless you were able to make the images a kind of "downloadable content" to view on a computer monitor.  

It would increase the download time of the iBook, but that also depends on how many images are in the book and what the overall finished and compiled file size is going to be.  

I personally have downloaded books that were 1GB directly to my iPad via wifi, but that is because I have fibre optic connection. Also I wanted that book! 

It comes down to what do you think people would want to go through to get your book. Trial and error. There is no "correct" answer for your question. Only advise.