You can never compare longGOP to I frame when dealing with compressed formats. I Frame formats like DNxHD, ProRes take up more storage in exchange for better performance and generational loss. Codecs like H.264 are traditionally used for distribution and not for editing and are optimized for a balance between quality and low data rates. I Frame formats will always be larger.
That being said, why not just edit the H.264 directly? While not as optimized as I Frame codecs, systems have come a long way to make it make it manageable (some NLEs more than others…).
But if you want to maintain “similar” level of quality for finishing downstream, you should even consider DNxHD 115 as a minimum (or work native H.264 till the end).
Michael