The main problem is that because the FAA IR puts emphasis on slightly different elements of instrument flying, most people find that they need a whole lot more than 15 hours to complete the JAA IR. I have heard of a couple of people who have needed the full 50 hour course, which has pretty much negated any benefit from doing it (other than having more instrument hours than would be the norm).
As for whether an FAA IR would affect sponsorships, well that really depends on which one you're talking about. CTC get a bit sniffy once you go beyond a PPL. I was considered a near miss on getting into CTC and was called back for a follow up interview, but when they discovered I had started the ATPL exams, they changed their mind on me, because they felt I had gone beyond the point they from which they could train me "ab-inito". Britannia were looking for "flight experience" so I reckon anything more than a PPL would rule you out of that one too. To be honest I'd just have a bash at the sponsorships while you can and not worry too much about things like IR's until later in your training.
Hope this helps
PW