It feels like the boss race against nolok is really too easy, I got him on the highest difficulty on the second try (in the first try I was so stupid to drive against the wall, and a bubblegum did the rest)
for me even the ghost race against suzanne in cocoa temple was much harder, this is actually the only race I finished at medium difficulty, which also feels a bit more difficult than the other normal races.
I would suggest to make him a bit faster, at least in the boss race, maybe make him using his hammer and bubblegum a bit more often, even make him put it down directly as a trap sometimes, I got him 6 times in a row, in the supertux difficulty, partly with a longer distance. I think on the other side the problem is, that he falls too often from that narrow part into the lava. 2 times I was in the goal and he was maybe just at the beginning of the castle.