Let us know how you get on. You might want to try it at 3200 as well as 1600 and then 800 to see what you get. I bought some expired P3200 which wasn't as old as yours but on the other hand it hadn't been stored in a fridge. I shot the roll at 3200, not knowing any better and found it very grainy, low contrast and quite frankly a bit disappointing but with only one roll that proves nothing.
You have plenty to play with and all it will cost is a bit of chemicals to find out its qualities.
pentaxuser