I suppose Henin could have won another two or three Majors, but I don't think I would call her an 'underachiever'. She had a very good career with the 7 GS wins, Olympic gold in 2004 and over 40 WTA titles. Justine had a good chance to win Wimbledon in 06 especially, but Mauresmo had her say that year. Wimbledon is the only big crown missing from Henin's collection, and that was one of her best chances to win, along with 2007. Surprising defeat to Bartoli that year. Henin was a match for the Williams sisters at her best in 2007, probably her best ever year.
One match I would have liked Justine to win was the AO final of 2010 after her comeback, but Serena just hung too tough. That would have been a fitting swansong to Justine's career, but all in all, she had a good run and not an underachiever in my eyes.