Roche's strength is unknown, but he can be assumed to be strong simply from his status as a Level 5 prisoner.
History
Roche's name was seen by Bentham as he searched for the name "Emporio Ivankov".
Trivia
Roche Tomson's name is similar to that of real-life pirate Roche Braziliano, a Dutchman who sailed in Brazil and was noted for his extreme sadism, even amongst other pirates. Also, Braziliano was at one point caught by Spanish forces, as was Tomson by the World Government, only he escaped and eventually disappeared.
His/her number on the list of prisoners in Impel Down is 6581.
References
↑One Piece Manga — Vol. 55Chapter 536, Roche Tomson's name on a list of prisoners.