You will notice the difference between an old (ie used significantly) LIPO and a new Lipo.
But with 13.5 and 5 min. racing in my view you are unlikely to notice any real difference between the major manufacturers. I have read posts about this battery having more punch than that one etc etc but I personally have not noticed any real performance difference swapping between the several different manufacturer batteries I have – better off prepping your tyres properly.
Your friend will of course say his new LIPO is awesome, he has to say that to justify the large bill he has just paid.
With the higher winds of stock motors 4000 even 3200 is fine; you are after all racing touring cars not flying a Spitfire.
And do not fall for the hype of the latest large C numbers. The output C rating of the cell has little bearing on the output of the cell in use, it only determines the cells maximum CONTINUOUS load, in our use this is totally irrelevant as we don't get anywhere near these levels, 30%-40% would be nearer the mark, if even that.
Think about how many amps you would have to draw to get 90c and then think about what that might do to your wiring if your ESC/motor could draw that load.