Port Ellen closed in 1983, so this is from the last year of it's run, which makes it quite collectable. Even if Diageo do reopen it the stuff will still be sought after as it's the "original" stuff.
In terms of quality; Port Ellen is good, but not amazing. The reason it is so sought after and valuable is not really it's quality but more the sentimental ideal or the stuff - Whisky drinkers are romantics and love a bit of "bygone era".
....don't get me wrong, it's lovely stuff, just over-rated and over valued.
Here is a Signatory bottling from 1983, although a tad more age on that yours:
if you could post a picture that'd help.
Looking about the value seems to hover around £350 ($530 at current rates) a bottle, or there abouts, which sounds about right for a Port Ellen of that age from that time.
Stocks of PE are getting lower and lower, so it's only going to go up in value. Whether you open it up though depends on whether you'd rather have a reasonable amount of cash or a lovely whisky from a time gone by (something you'll probably never buy again). What's worth more to you?
If it were me (I'm a romantic and I don't need an extra £350), I'd crack it open. It was made to be drunk, and when was the last time you quaffed something from a legend?