From a PM answer of James Hooker, about my question about buffer gas stating of the modern T5 HO/HE on
his site , It turned out that the gas of the modern T5s, is just a pure argon, just like the classical T5 4-13W, the T12s and T8 of 15W, 20W, 30W and 50W (So they have the same starting voltage as the old fluorescent lamps), and that the reason for their higher efficiency over short T5s, is because the pressure of the mercury is kept nearer optimum levels, due to a long mount at one end (The reason why in long T5s, one electrode is longer than the other one, causes larger blackening in that area, when the tube is old.
Also James said me that the red and green components of the triphosphors are usually better than the triphosphors of the T8 (Probably the reason why current triphosphors T8s are barely brighter than halophosphors T8s).
There is also less self absorption of the mercury 253.7nm UVC line, because of the narrower tube diameter.
If this is the case, so why the optimal temperature of the modern T5s is 35degC (95degF), and not room temperature? (In room temperature, they are less efficient than the T8s).
And of course, the reason why they are limited to electronic ballast, is because manufacturers refused to produce magnetic ballasts for them, because of the time of their invention (14W and 21W HE T5s can be operated successfully on a preheat/switchstart magnetic ballast for 13W classic T5s).
Edit: And why the PLs, CFLs and 2D lamps have argon-krypton mixture like the T8s and not pure argon like the modern T5s?