User blog comment:B214/Volume 23 discussion blog/@comment-31764731-20170825182223/@comment-29963364-20170829190816

It's not that his Boosted Gear increases his Demonic Power, it just makes it more powerful.

Let's say Issei has 10 units of Demonic Power, and an average Ultimate-Class Devil has 150,000 units of Demonic Power.

When Issei uses [Boost] and/or activates his [Cardinal Crimson Promotion] (which we know makes him Satan-Class), his Demonic Power doesn't increase from 10 units to 200,000+ units. He still has only 10 units, but the Boosted Gear makes those 10 units just as powerful as 200,000.