Clearly since you haven't done your research and are just B.Sing, i wont waste my time my crafting a huge response. I will cheat a bit and post a very beautiful analysis from another website, hope it open up your eyes because my friend you are living in a lie if you think a missile that big and travelling at that speed could have a low IR signature.
"The problems with running very high speed bodies through dense air are manifest. The main aerodynamic issue we found was with a form of pressure drag that created buffeting on the airframe. Basically what happens is the kinetic heating on the missile body causes the air passing over it (the aerodynamic boundary layer - as opposed to the atmospheric boundary layer between sea-surface and air) to expand rapidly. This expansive air intersects with the airflow over this missile and induces drag.
The denser or, rather, wetter the air you fly though the greater this drag is as the greater potential energy transfer from airframe to air. Furthermore the quicker you try to go through that air the higher the thermal loading on the airframe and the more intense the pressure drag. You can calculate the average effects from this condition but those equations were ones I last studied over 14 years ago and are a bit vague now.
There are solutions to this, of course, heavy metals in the missile fuselage to act as a heat sink being the most obvious but then that adds weight and changes your size, propulsion, performance, range and payload calculus. The most obvious, and the one seemingly adopted by the Moskit design team was to fly slightly higher and slower to reduce the loading.
As to the issue of subsonic warhead damage look at the bows photo of the USS Stark again. That hole in the superstructure and the heavy list to port was nothing to do with a fire - that is damage from the impact!. That damage is sufficient to send the ship back to port. If the prevailing conditions are kind it might even make it!. Even if the ship does make it back to be repaired its out of action for duration of most modern conflicts. I ask you again how much damage do you think you need to do to a ship?.
You say Brahmos has a low RCS design. From a look at the airframe I dont see it personally, at least if you compare it to a real low-RCS design like NSM, but even if it were the case you cannot propose that Brahmos has Low Observability characteristics. Not when you are talking of such a large missile travelling at such a high velocity. Even basic IRST's like Radamec's 2000 series can detect tactical fastjets at 20km plus, Thales's new SIRIUS sensor has, allegedly, the capability to detect TBM's at ranges in the hundreds of kilometres. A mach2.8 missile travelling at altitude will beacon on IRST at 40km even if, and IMO this is very unlikely, the radar doesnt catch it. The basic physics of it is that a vehicle cannot expend the kind of energy that M2.8 requires without radiating some of it out into the environment somewhere in some form."
? 
