Some very good points there. Note the D6 has LESS shielding than the D5, and a worse power curve. Actually, most players would take a D5 over a D7. The D5 is a war cruiser, so really the Klingons, as with other races, skip CLs altogether. They don't actually have ANY. Upon examination of your arguments, and looking at the Hydrans as a common foe with similar ship production, the D6s are like unrefitted DGs -- poor cruisers that are best used as hulls to build variants from. Still, for pricing and availability, I would rather see a D6 model in the CL class on the D2. That's certainly up to the admin running a campaign, so it need not be addressed in a common shiplist. However, D6s and KRs should be classed the same, whichever way is chosen.
This is exactly why I like these threads -- people can discuss things and find common ground or improvements without getting into a flamefest.