How long would you expect a heavy ale to keep improving in the keg? I'm drinking a wheaty ale I brewed almost 13 months ago, and it seems even better now than it was early on. Beersmith thinks it's 8.6% ABV.
Wheats are usually better young, and at 8.6 I'd really expect it to start declining at 5-6 months. Of course grain bill and hop levels make a big difference, so I'm just throwing out a general guess. Good that it's holding up and even getting better, though. Grains like Brown malt get better with age, so that can affect it, too.
"heavy" is not a character generally associated with wheat beers - nor is 8.6% ABV - so I'm betting there's something unexpected under the hood here, in which case there's no telling where it's going...