There is a six speed... I almost bought one. The only reason i didnt, is because they are rare as hens teeth, therefore the support for them would be worse than my current C52.
Both come with the option of LSD and non-LSD. From Memory, the C52's are LSD, the C54's are not and the C56 (6 speed) come without, but have the option of putting one in.
The six speed is also alot shorter ratio's than the 52 and 54, but doesnt have as good of a top speed. The 5 speeds on the other hand are too tall, except in 3rd, which is perfect. Strange really.
Although, they can still allow you to smoke a 2.4L I6 N/A Lancer