As usual Wikipedia is only half right ... the story of the boat tails is largely relevant to the No4 2 groove barrels specifically - not so the other numbers. The facts of the matter have always been that they were not designed for the SMLE - only for the machine guns and the hot load is certainly one of the issues. Barrel life was said to be halved using the MKVIIz and MkVIII ammo so I'd stick with the British Army specs rather than Wikipedia.
Here is the real reason why ...
Caution should be exercised when considering the use of the MkVIII round though, especially in older Long Lee’s or SMLE’s. According to Labbett & Mead's ".303 Inch" ballistics for the MkVIII it is listed as 20-21 tons/sq versus 19.5 for the MkVII with a muzzle velocity of over 2500fps as opposed to the 2440fps of the MkVII. At 2240lb/ton, this means pressures of up to 47,000psi, which is chasing at the heels of, if not breaching, current published safe pressures for the Lee action. I must thank Dr. Justin Moretti for bringing this potential hazard to my attention.