The IR content is taken from the ATPL course so much of what you study is identical. Depending on whch of the 7 exam topics the question bank used will be a varying percentage of the ATPL question bank. If I did it again I would do the complete ATPL theory course otherwise there would be a lot of duplication unless you only want to go as far as the IR. Under the new EASA IR there will be a significantly reduced question bank so this may also be a factor in your decision if you want to stop at the IR