YES
O(n^4)
TRS:
 {
                   concat(leaf(), Y) -> Y,
               concat(cons(U, V), Y) -> cons(U, concat(V, Y)),
               lessleaves(X, leaf()) -> false(),
      lessleaves(leaf(), cons(W, Z)) -> true(),
  lessleaves(cons(U, V), cons(W, Z)) ->
  lessleaves(concat(U, V), concat(W, Z))
 }
 DUP: We consider a non-duplicating system.
  Trs:
   {
                     concat(leaf(), Y) -> Y,
                 concat(cons(U, V), Y) -> cons(U, concat(V, Y)),
                 lessleaves(X, leaf()) -> false(),
        lessleaves(leaf(), cons(W, Z)) -> true(),
    lessleaves(cons(U, V), cons(W, Z)) ->
    lessleaves(concat(U, V), concat(W, Z))
   }
  Matrix Interpretation:
   Interpretation class: triangular
            [0]
            [0]
   [true] = [0]
            [0]
   
                [X7]  [X3]    [1 1 0 0][X7]   [1 0 0 1][X3]   [0]
                [X6]  [X2]    [0 0 0 0][X6]   [0 0 0 0][X2]   [0]
   [lessleaves]([X5], [X1]) = [0 0 0 0][X5] + [0 0 0 0][X1] + [1]
                [X4]  [X0]    [0 0 0 0][X4]   [0 0 0 0][X0]   [0]
   
             [0]
             [0]
   [false] = [0]
             [0]
   
          [X7]  [X3]    [1 1 0 1][X7]   [1 0 0 0][X3]   [0]
          [X6]  [X2]    [0 0 1 1][X6]   [0 1 0 0][X2]   [0]
   [cons]([X5], [X1]) = [0 0 0 0][X5] + [0 0 0 0][X1] + [0]
          [X4]  [X0]    [0 0 0 1][X4]   [0 0 0 1][X0]   [1]
   
            [0]
            [0]
   [leaf] = [0]
            [1]
   
            [X7]  [X3]    [1 0 0 1][X7]   [1 0 0 0][X3]   [0]
            [X6]  [X2]    [0 1 0 0][X6]   [0 1 0 0][X2]   [0]
   [concat]([X5], [X1]) = [0 0 0 0][X5] + [0 0 1 1][X1] + [0]
            [X4]  [X0]    [0 0 0 1][X4]   [0 0 0 1][X0]   [0]
   
   
   Qed