Is Chess Rating linear?

or not sure the term, is it hyperbolic?   Is 2600 player same strength over 2300 player as

1700 player over 1400?   (both seperate by 300 pts)  

In theory, a 2600 should have the same score in a multi-game match against a 2300 that a 1700 has against a 1400. That's how the rating system works. In practice, who knows? A 2600 probably didn't obtain his rating by consistently beating 2300s; more likely it was by scoring around 50% against other 2600s. The same goes for the 1700.

A 2600 may maintain his rating by scoring roughly 50% against peers, but that's not how a player would obtain a 2600 rating.

A future 2600 player, who may be rated only 2000 at the time, will at some point need to consistently be beating 2300 opposition.

Basically yes.

But (more or less) all learning curves are exponential. You make the most progress in the beginning. Going from 1400 to 1700 is much easier, and faster, than going from 2300 to 2600.