I just recently (and painfully) donated my Blue Hippo Tang. I've had it for 6 months. I bought it when it was 1.5" big. Six months later, it's 3.5". And I am starting to see some dangerous signs of HLLE. The "forehead" (if you may) is starting to remind me of a Parrot Cichlid. The "top" fins are looking great, except that towards the face, it looks like it is receding.
I was a stubborn guy wanting a Blue Hippo on my 4ft 55G. But you have to realize that the fish's world is not just from left to right of your tank. The width also matters. And a 4ft 55G is just too small for an individual. Everytime it turns around, it makes me crunch because it's just too small.
And you are right, TS, people promise to upgrade or donate when it outgrows but you get attached very very much. And it is much painful to donate a Blue Hippo Tang 4 - 5" long. On top of that, they usually give you 1/3 of the selling price. And they sell it at around $90. You get $30 for it....it's sad financially as well.
You've had all the warnings, it's up to you now. But here I am, a sad ex-BHT owner.