Fuel consumption has nothing to do with HP. Yes, I'll agree that the R will do the same work as the 4230 in DOUBLE the amount of time on 2/3 of the fuel.
Slippage is simply a factor of operator error of improper ballast. Both tractors need weights in accordance with slippage.
According to Nebraska the R made 43 Drawbar HP pulling 6600 lbs in 1st gear and the 4230 made 83 Drawbar HP pulling 11,100 lbs in A2. These guys know a lot more about slippage and measuring power than either you or I.
50 horsepower is only capable of half the work of 100 horsepower. That's just the hard facts.