Both flour power and motor wattage are products of the marketing department and are best ignored when dealing with practical limits of the Professional 5 Plus and Professional 600. I do not believe that the Professional 600 has any more bread dough capacity than the Pro 5 Plus. On the replacement parts market, the same motor is specified for both mixers and I assume that the same motor is used on the production line as well. It is a nice, competent 250-300 motor that is tortured into outrageous maximum wattage numbers so the marketing department can have its say.
The bowls or both mixers are the same shape with the 5 quart bowl a bit more than ½” shorter. Since I seldom use the top half inch of my six quart bowl when baking bread, I believe that the additional capacity of the big bowl does not translate into anything usable when mixing bread.
My old Professional 6 is about to have its fourth birthday and have found that as long I keep the flour amount below 1kg\35 ounces I can knead any dough to completion as long as it is hydrated at 62%-65% even if I want to knead for 8-10 minutes. With multigrain and\or high proportions of whole wheat, I find that the mixer is most comfortable with 28-32 ounces of flour.
Whenever I have tried to exceed these amounts, the strain on the mixer increases very quickly. The dough starts to escape and if the dough nears full development and has significant amount of gluten, the dough ball can almost stop the mixer in its tracks.
In practical use, 40-48 ounces of flour is more than is prudent regardless of what the marketing department says about the theoretical maximums of the mixer and which mixer you are using. I am also convinced that a mixer does not necessarily “wear out” with heavy duty use over a period of time. Most problems are the result of a single catastrophic event of overload.
I decided many years ago that preserving my mixer was more important than getting the most dough possible out of it per mixing session and have been rewarded with many hundreds of loaves of bread with no discernible wear on the mixer when the gears are examined.