Here you are forgetting the basic concept of Stand off weapon. Stand off weapon means any weapon which could be fired from a safe distance out of the range of enemy air defence. So even with its current range of 300 km, the launching platform (Su30) would be effectively out of range of any current SAM except 40N6 of S400. So whether you want to extend its range as an standoff weapon or not, depends on how you want to use it. For this very reason I was saying that India should go for some heavy dedicated bomber instead of modified fighters for CM. But lets not discuss it now.
You're being very simplistic and incorrect in your view of the battlefield. Just because the aircraft is out of range of the SAM, it does not mean your missile will hit any arbitrary target with whatever piddly range the stand off weapon has! Imagine S400s deployed as a fence and your target is 300 kms behind the air defence, then your stand off weapon needs to traverse 600 kms. Extended range is always useful (especially if it comes for almost free). Kapsich?
Now this half filled tanks and all are just plain bullshit. If someone has really written something like that, its nothing more then some ignorant writing. Look at how things work. Engines in a missile or rockets are used to provide thrust or force to the body. Now if we look at the relation of Force, Mass and Distance then a simple layman equation comes up as,
F=MxD.
Or lets say for sake of simplicity, if you have to apply a force of 1N on a mass of 1Kg it moves by 1M (just for example). Now in case of Brahmos, if Brahmos weighs at 1Kg, to move it by 1Km the engine apply a force of 1N. Now if the fuel tank is fully filled its mass would be doubled. Then how come 1N of thrust would make it move by 1Km? You have to rework on the engine.
With increase in mass the velocity and range of the missile would be highly decreased if engines are not reworked.
Dude!! Maybe we should talk after you graduate from high school?? In your view of the world, the weight of the missile is constant throughout its flight! Where in fact. the weight of the missile changes constantly during the flight ( I am hoping that the light bulb went off in your head - that's right, the fuel quantity is decreasing). Despite this change the cruise missiles can maintain steady speed. How do they do that? You're right again - the engine can control thrust! Another thought exercise: Tomahawk cruise missile flies at subsonic speed for nearly 2,500 kms, now by your logic, during the last 100 kms of the journey the missile should attain hypersonic speed because the engine thrust remains the same but the weight has reduced drastically. You see what's wrong with that picture???
As I said before most of the supersonic cruise missiles developed by Russia have around 600 kms range; and all their non-MTCR export versions have a truncated speed of 290 kms (be it with India or Indonesia!!).
Of course, you don't have to believe any of the above. I hope you atleast know how long it takes to design a new engine and a cruise missile (min of 5-6 years; on an average around 10-15 yrs). Just wait and watch how soon India is going to declare the extended ranged Brahmos as being operational!
Also, I didn't want to mention that earlier but F=ma; md is not a physical attribute at all . Your confusions maybe caused because you may still be in elementary school, where Physics hasn't been taught as a subject by itself; you're probably learning stuff as 'science'!!