90% of these are released as mods, right here! Quality varies, but overall most are still very good. Some are conversions from other games such as iRacing, rFactor, etc.
The Isle of Man TT in particular apparently caused some issues do to its shear size, but I guess they some how made it work. Admittedly, haven't even driven a full lap, and have crashed many times, but I feel it is as close as we can get.
The IOM in AC is converted from GPL, so the surface mesh is a bit off in some places. Also, it's an abridged version of the full track, only at ~30km instead of the full 60km. Most of the important corners are all there, it's just the straights that are scaled down in some places. Still a worthy track to have in your collection, until Kunos laser scanned the real thing (who knows when lol).