Uh, no, Issei wouldn't reach Galaxy Level with 20 Boosts. And if everyone would actually read everything presented in the threads rather than basing their arguments off of incomplete information, I believe the conversation would go much smoother. For now,
I'll specifically show examples of this. And Issei rarely uses more than 14 Boosts at a time in his later forms (IMT/CxC) because his body wouldn't be able to contain the power, in DxD/PDxD, he only ever uses 1 Boost. Though 20 Boosts in his Base are easy.
Edit: Y'all are acting like it's ^2^2^2^2
Also, perhaps a statement like
this was what you all were referring to? Just in case this statement's relation to the Boosted Gear is questioned, I'll explain why it specifically refers to the powers of the BG. Firstly,
the mention of miracles have almost always been portrayed as referring to the BG (And by extension, Issei/Oppai)/Sacred Gears. Second, narratively, it'd make sense for Saji to have jealousy toward Issei's Boosted Gear (Also shown via the subtle comparisons between the weakest Dragon King, Vritra and the Strongest Heavenly Dragon, Ddraig throughout the volume and Saji's acceptance at the end of their second fight that he's still strong.) and an understanding of it being a major source of his strength, after all, far before this scene (during volume 5),
Saji's plan during his fight with Issei was to specifically stop him from Boosting, not from using his BxB or anything else. Third, when it comes to basic power increases like the ones that Saji was referring to, the most notable one in reference to Issei would be Boosts as it's his and the Boosted Gear's main ability.