I have contributed to my IRA (today) towards my 2016 taxes & will indicate such on my 2016 return.
My updated form 5498 may not arrive before May '17
Will I have to amend my return ?
No. Contributing to your IRA for 2016 in 2017 (up to the due date of your return) will not require you to amend your return.
Form 5498 is an informational return for your use, and is not needed to prepare your tax return.