User blog comment:B214/Question and Answers 6/@comment-190.164.227.239-20161201033222/@comment-29963364-20161205094033

["In terms of overall balance, the White one is superior by several folds. But in certain aspects, there are parts where the Red one is superior. In terms of simple attack power and direct speed, the Sekiryuutei wins."]

This is Sun Wokong comparing Issei and Vali in their normal Balance Breaker armors.

["Sekiryuutei's ability is to boost and transter the power while the Hakuryuukou's ability is to divide and consume the power. ---This is the so-called ability of the Longinus "Boosted Gear" and the Longinus "Divine Dividing" since ancient times. The fact is, the past hosts of the Two Heavenly Dragons fought while using these abilities. There were those that relied on the usage of their ability to fight in a unigue way......but there wasn't a single one of them who are going through unrealistic power ups like you too, you know?"]

This is what Sun Wukong said to Issei and Vali during their training session in vol.17.

From what he says, the past hosts of BG and DD only ever fought by using the abilities they started with (boost & transfer and divide & half-dimension) and the Juggernaut Drive. While some used these abilities in unigue ways in order to get stronger, they never went beyond that. In the entire respective histories of the BG and DD, no host has taken their Sacred Gears, and the powers of the Two Heavenly Dragons to the same extent as Issei and Vali (both reached a form beyond the Juggernaut Drive and unlocked the lost ablities of Ddraig and Albion).

So Sun Wukong is basically saying that Issei and Vali are the strongest Red and White Dragon Emperors in history.