Had a situation like this myself early in my brewing days when I started brewing bigger beers and nothing I did helped (adding yeast did nothing as the sweetness was coming from unfermentables in the beer which sucked, and adding a "tea" only made the beer taste even weirder and more out of character than I was hoping it would).
I gave up and ended up bottling it and giving it to the folks I know that are my "Mikeys" drinkers of my beer (they will drink anything as long as it gets them drunk).
Were I in the same boat now (and knowing more than I did then), I might drop it on some oak to better tolerate the cloying sweetness which is not something I tried when I had this happen to me. If you do this you will want to package it and let it condition for a while..Imp Stouts take a bit longer to come together when packaged/kegged than your normal ales (Blondes/IPAs) do. Time is your friend with this.
Next time I brewed this batch, I made sure I mashed a little lower (150'ish) and pitched much fresher yeast along with more cells to dry it up a tad more.
Topped it off with some bourbon soaked oak cubes and it was the best RIS I have ever had and its currently in my normal rotation for beers I keep on tap when the pipeline is full.
:fro: