How to increase the serial port's data throughput
If you are looking for throughput then stop sending the ASCII conversion of numbers and just send the numbers.
In other words, use Serial.write() rather than Serial.print(): you can send ~25k integers per second with a 52k baudrate.

As you are actually remapping the ADC output to a single byte, that means you can easily achieve a 48k/s throughput with this simple change.

If you are also looking for low latency please consider the USB connection is actually packet based, not stream based, which means you will receive the data in chunks, no matter how frequently you write. You can speed it up a bit flushing the channel, at the cost of throughput.