Okay, this is basically just a Copy & Paste from a post debating the topc on the Excalibur forums; I just had inspiration...
Using the information on Memory Alpha, this is pretty much as I expected and found the TNG and TOS warp scales to look like.
(Note this is a cheap graph I made online.)
My theory on Warp Factors and Transwarp (Excelsior style) is that a Warp Value is equal to a certain amount of energy exerted on the dilithium crystals (Or mass of matter + Antimatter being used) And this corrolation does not change between TOS and TNG, it is how the result energy is handled that accounts for the speed change. The USS Excelsior was an experiment into more efficient usage of the post-warpcore energy; it obviously yielded positive results, but may have been too complex to mass produce in the era, as well at refit all the ships in service.
By the year of 2293 (The Undiscovered Country), when a large number of Starfleet vessels were retired/mothballed due to a cease of hostilities with the Klingon Empire, it was more viable to refit the remaining number of ships (Which would be quite few) with the new drive systems, due to less resources being used in defense and a re-focus towards exploration; as well as a possible advancement in engineering/production technology.
*Takes a deep breath*
Thoughts?