First of all understand few things...........
As of now BRAHMOS is a 290 km range missile. Only one test for extended range been done so far as per media report in March 2017. But interestingly, even BRAHMOS co doesn't publish it.
http://www.brahmos.com/press-rel.php
It implies that it might or might not have been specifically tested with different engine or fuel. It is simply absurd to say that the range was limited by a half filled tank.
The range of BRAHMOS has been limited by its flight program. You simply can't program it to fly beyond 300 km range. In its current form it might fly beyond this range, but it is not something achievable overnight. With the deployed regiments and batteries, it would mean a complete overhaul of the missile. Being a cannister based missile, this gets even more complicated. Range would be increased eventually, but gradually. As of now don't expect the current lot to fly beyond 300 km.
As far as mastering RAMJET engine of BRAHMOS is concerned, India has not yet done it. Work and study is in progress.