I run 12/50 on the same bike. I've never run the stock 12/49 either.
I think going to 12/53 isn't going to do what you want to. Yes, you might be able to pull 3rd through some corners that you run in second now, but each gear is going to be that much shorter.
On my bike I have to enter most sharp corners in second and immediately shift to 3rd upon exiting the turn. If it's a long sweeper, I have to shift to 3rd midway through the corner.
I would try 12/50 and 12/49.
Finally, I don't think you'll get much low end out of your engine no matter what you do, mostly because it's a 125. The best way to broaden the powerband and add some low end would be to go for a 144 kit. No replacement for displacement.
One other consideration is a flywheel weight. This will smooth out the power and make the powerband feel broader. I haven't tried one, but most who have rave about them.