Jamaican overproof is dominant flavor, even with only 1/2 oz. Others note that letting it rest (and get a little melt-water) helps mellow it out - I think I'll make with cube ice next time. Also, PX sherry adds sweetness and a nice flavor, but not as much sweet as the demerara syrup I use in a Rum O.F.
My personal notes say this is brand-dependent, and I'm sure you'd agree. Appleton 8 and Hamilton 151 was serviceable at first, though as the block of ice melted, it vastly improved. Something to play with.
It is boozy and dry. Definitely, a cocktail that needs to rest and let some of the block of ice melt. First comes over like firewater, but afterward becomes a very nice sipper. It is definitely a 10 or 10+ on the boozy scale.