While the Chiron's engine is undoubtedly more efficient then the Allison, there are some more considerations. The vast majority of piston engine aircraft are rpm limited to 3000rpm or so to prevent the propeller tips from entering the trans/supersonic region.

Automotive engine designers often increase rev limits as a strategy for increasing horsepower - in the Chiron's case it redlines around 6k and generates peak HP, if I had to guess, at around 5300 or so. The Allison would have been limited to about 3k, and as the other poster has said, would be expected to run near peak rpms for longer durations.