It is 81 Marquise, not 243
3 Marquise = 1 Imperial
3 Imperial = 1 Flawless Imperial (9 Marquise)
3 Flawless Imperial = 1 Royal (9 Imperial or 27 Marquise)
3 Royal = 1 Flawless Royal (9 Royal or 27 Imperial or 81 Marquise)
so 1 Flawless Royal needs 81 Marquise to make.
If you are looking to augment something, then you need 3 max gems (Flawless Royal) which would take 243 Marquise.
Yes, you do. For a lot of people with OCD, you gain sanity. It is the same reason people will spend 20 billion gold and mats to get their gloves from 49% crit damage to 50%.