Fast and small may or may not produce a larger impact force than slow and big. It depends on the relative differences in mass and speed between test subjects. If the difference in mass is relatively greater than the difference in speed, it's better to be big than fast. If the difference in mass is relatively smaller than the difference in speed, it's better to be fast than big.
What's absolutely incontrovertible is that 230 lb (104.3 kg) TR moving at an assumed impact velocity of 8.3 m/s (based on 4.4 40 speed) produces 3,592.6 joules of energy, while 350 lb (158.8 kg) DF moving at an assumed impact velocity of 7.5 m/s (based on a 4.9 40 speed) produces 4,466.3 joules of energy. In fact, if DF's 40 time is as slow as 5.3, his energy output still exceeds TR's. He has to slip to 5.4 before he and TR produce roughly equal output. Am I arguing that DF's a better back? Absolutely not. He's not even on the same playing field as TR. But we're not talking about who's the better back. We're talking about what's better for producing impact force - size or speed. No matter what, it's always better to be big and fast.
I agree that what is important in calculating force is the speed at actual impact, not an average speed over 40yds, which as you say, will range from zero when the ball is snapped to some velocity greater than the average. Do you have any suggestions for a better approximation for impact velocity? Without a radar gun pointed at a target on impact, I'm not sure there is one.
It could be legitimately argued that because of TR's lower mass, his acceleration (and therefore likely initial impact velocity) is much greater than DF's, narrowing or even overcoming the gap in energy production.