My tig outgrew my 8ft 300 in about 4years. His body got to 26inch and his long streamers eventually started to rub the other side of tank when turning.
It's not the gallons..it's all about the dimensions with tigs. They don't need a ton of swimming room so a short but wide tank can work imo with even less than 300gal if he has proper turning radius for all his body parts..I would say no to a tig longterm unless he's got plenty of width to turn or you have future plans to upgrade