Though, this does bring up a point I'd like to make; A modest nature is actually extremely viable in Ubers. To give you a good taste of how fast Mewtwo is, a modest, maximum speed Mewtwo will still outrun a timid, maximum speed Latios.
No it isn't. Darkrai, Shaymin S, and Arceus exist. You also speed tie with other Mewtwo as well. 10% more SA is nice but outrunning EXTREMELY common ubers is more useful.