I still believe in the power of short range leapers. The Mammoth should be equal to an Archbishop/Cardinal on the 8x8 board, and to a pair of Bishops on the 10x10 board [Edit: also equal to Rook + Pawn]. I just read the comment below by Mats Winther. Simple 'pawnless endgames' are a good test of piece strength.
For an index to Ralph Betza's work, see About the Values of Chess Pieces -- Contents. His early attempts at STANDARD values (including Ferz, Alfil, and King/Commoner) can be found here.