I'm comparing the same GCW when referencing my F250 verses 5500. The 5500 is about 2500 lbs heavier than my F250 when empty, so a GVW comparison was too hard. I had the PowerStroke long enough I know how heavy I was and what type of performance it delivered. The DRW on the 5500 doesn't help performance, but the 6-speed with 4.44 differentials are an advantage over the previous 5-speed with 3.73 differentials.
The only reason for my change was to get more payload. I would have had the chassis cab tune of 6.7 PowerStroke if I had chosen the F550 at 300 hp and 660 ft-lbs. This may have been a let down compared to what I had been used to driving.
If someone has a 8-10 year old 5.9 Cummins and was going to the larger chassis cabs, they might also have similar experiences to me. If they own an older one, they may only have 215-245 hp and it would be a noticeable improvement.
I don't expect breakneck acceleration, but it is nice to get to speed faster on a freeway on-ramp or when you need a pass a slower moving vehicle. This is where I saw the my older PowerStroke better the Cummins. Others coming from a V8 GM diesels will most likely see the same when going to the I6.