1) Instosis probably wins about 65-70% of its games against FGs. It's not nearly so good against the arena, due to the massive amount of explosion/steal there. RoL/Hope has a -much- lower win rate than that. The only problem with Instosis is that it's about as exciting as watching paint dry. Rainbow Control is way more fun, but you won't get much more than 50% with that, but I use it anyway most of the time. I've seen a few decks claim over 50% win rates, but the only ones that actually have 50% win FG rates that I've seen are Instosis, I've GotP Time, and Rainbow Control, with Instosis having the highest and rainbow control the lowest win rate of the three. SPlat might be a 4th, but I haven't tested it against FGs so I can't say either way, though it's definitely good in the arena.
2) Some of the percentages for those decks look rather high to me (two of them in particular, but I won't insult the creators of those decks by naming them) but most of them probably win at a rate roughly around what they claim.