mastercomsI feel like it's a misunderstanding of a few variables as well as possibly older FPS configs not working as well:
First, I think the main premise of this was because people associate DX8 with lower quality, while it is much more than that. It changes the entire rendering pipeline to something which doesn't work as well on newer GPUs. The speed boost that people see at all is simply because DX8 supports less features, and thus renders less, but it renders less at a lower efficiency.
Furthermore, many configs in the past did not properly save changes to TF2's video config, which caused the game to reload many times on startup, causing graphics systems to initialize in pathways which could have resulted in glitches and lower performance. mastercomfig saves the graphics options that you select to the video config, so that these reloads are avoided. To completely ensure that this is the case, use modules entirely instead of individual rendering commands (usually prefixed with mat_, also applies to r_rootlod).
Finally, I think that in some cases, people found that their game ran better without a config, and I don't want to disparage the work other people have done, but perhaps people associated a config with only changing graphics quality, while it does much more than that, and thus, can affect the game's performance beyond lower quality graphics.
I feel like that information about DX8 should have been more wide-spread! If it was i completely missed it.