We're talking two vastly different classes here though...
This isn't the difference between GT Pro and GT Am, this is DP and GT, two vastly different classes. Having one DP guy running the same times as GT creates a ripple through GT, wherein the GT then have to fight not only themselves, but a car superior in a straight line and certain corners, or at least is supposed to be...
Its bad enough knowing we GT guys get out of certain corners faster than DP, making even the all-too-common lapping a noteworthy concern. To navigate through GT traffic is a skill, as is being able to fight for position AND allow DP to lap safely. Imagine having to do that constantly? Can only raise the bar so far...
What if the slower driver of the faster class is barely as fast as the faster drivers in the slower class? What then?