User blog comment:B214/Volume 22 spoilers/@comment-95.90.227.198-20170201191840/@comment-30924344-20170202181734

The other 4 evil pieces will most certainly become mutation pieces due to DxD's and Ophis' powers. His power is certainly going to surpass the Heavenly Dragons and Vali. Vali just has Albion's and his own powers, whereas Issei has Ddraig's, his own, Great Red's and Ophis' powers. He has the power of 2 gods inside him. It would be pure bullshit for Vali to be equal to Issei at their mastered powers; Vali did not get Ophis' power btw to activate his DxD mode. she just blessed his and Albions powers together, which is why Vali's and Issei's DxD modes are equal. Issei at full mastery of his powers will be 2x stronger than Vali because 2 more power sources than Vali.