I'd agree that six years would be a reasonable estimate at this time. That being said with the current design of LED fixtures the energy savings will likely never give a payback if you have to essentially replace the fixture every 6 years. That's without my concerns of premature failure which seems fairly likely with what I've seen with current fixtures on the market and the lack of a decent warranty period on newer fixtures.
On the topic of par vs spectrum I don't believe you're correct, just because the spectrum of the bulb changes doesn't mean the par will also change. Certain halide bulb and ballast combinations will last far past the year mark in terms of par while spectrum is a separate issue.
http://reefkeeping.com/issues/2007-04/ac/index.php
I see LED taking more of a role in supplemental lighting in combination with halides or some other type of HID lighting of the future.