Most published stats for tech tree vs premium tanks are comparable. Stats like aim time, aim circle size, hp/ton, and top speed are all comparable, with armor being the only obvious advantage for premium tanks. The real advantage is in the hidden stats.
Gun accuracy – Excluding light tanks, there are 23 tier VIII tanks with a moving gun dispersion of .15 or less. 16 of those 23 are premium tanks.
Mobility – Again, excluding light tanks, there are 34 tier VIII tanks with a medium terrain resistance less than 1.10. 24 of those 34 are premium tanks. 8 of the top 9 are premiums.
This is the hidden advantage you gain when paying for a tank.
Disclaimer: I excluded lights because just about every tier VIII light met the criteria listed above