It depends on how bad you want it and how hard you train and the type of training you are getting.
You can definitely train the brain, just like how anyone can develop and increase body muscles and endurance. The limitation would just be how far your brain is capable of.
An example would be, off the bat you can run 100m in 12 seconds. through gym work, dedication and better technique maybe you may get to 11 seconds.
But then is it possible to get to 10 seconds ?
Its finding what your limitation is, maybe its endless, but you dont have the time.
Before the "boost", I heard that many titled players couldn't break 2000 standard, and computer impossible was like 1800.
Daily ratings are by far the least accurate ratings, and tend to over/underestimate drastically.
There's a 2600 GM with a 1400 Daily rating (even when he wasn't timing out, he was around 2100), and 1400 USCFs playing 2-3 games at once who are 2100.
Are you sure that GM isn't at 2783 FIDE?. There was a fantastic GM that timed out all his games a few days before the Carlsen-Karjakin WC match. It looks like he couldn't spare time for his daily-games around WC-time. Before he timed out he won tons of games (232?), had 8 draws and 0 losses.
Those 1400-players that plays Daily at 2100 are very good at chess, but they need more time to see all the tactics than available in otb long chess. There also is the explorer we can use in daily, which takes away most opening errors, and partially evens out the odds between the 1400s and the really good players. Players that plays Daily at rapid speed will not perform at their own maximum. some players does that. Not to win, but to get practice and test out openings and ideas.
I think the Fide-rating for long chess is the most precise scale.