I think you'd be better off asking here -
https://www.reddit.com/r/BitcoinMarkets/Bitfinex and paying tax on trading profits are very regular subjects in the daily discussion thread.
It's going to be very interesting to see what happens to USDT on Bitfinex.
As it stands there's no way they'd convert a USDT balance to USD without informing anyone, that's if it's possible at all. That would open up a whole can of worms about liabilities and compliance. It'll stay how it is.