Has anyone tested to see how impactful pre-loading chunks is on tps/mspt at varying amounts of players? I know it makes the server lag less, but I wonder if it has been tested?

It doesn't directly impact TPS/MSPT since 1.13 as world loading and generation are done on a separate thread (not the server thread). So your answer is there is "no server performance impact" technically speaking (in a void where you only care about the server thread and TPS/MSPT impact).

Practically speaking, yes, pre-generation will improve chunk loading performance as the chunk needs to be generated first otherwise. This usually means smoother gameplay with less waiting on chunks, especially when moving quickly through the world in what otherwise would need to be generated on-demand. If you have a server with a large player count you will notice this, and the struggles of starting a new map where nothing is pre-generated is extremely noticeable and the difference matters a lot (even though it doesn't affect mspt at all, the game will "feel" laggy).

I hope that answers your question from a general sense. We are on Discord if you want to discuss this in more detail.