I would love for it to accomplish that, but I don't think GotG2 can keep up with GotG's late legs. GotG has the best legs for a modern superhero movie. Best legs in over 20 years for a superhero movie with 40M+ OW.
And it very far behind "that pace" right now. GotG made 8.1M this weekend compared to GotG2's under $5M weekend. GotG2 won't even make as much over this entire week as GotG made over this weekend alone. Even though GotG2 has the benefit of summer weekdays and Father's Day to help it.
You say it could outpace GotG with help of a small holiday like the 4th of July, but GotG is far ahead this week, had virtually no competition for months, and held on to its theaters much better. Meanwhile GotG2 is in the middle of summer facing strong competition.
I've said it before and I'll say it again: GotG is not a good comparison. The differences between it and GotG2 are just way too big. Any MCU movie released in the first weekend of May should be a better comparison. Iron Man is probably the best comparison if you want to make a reaslistic prediction for what the ceiling for GotG2 is right now.
I don't think $400M is completely impossible, but it looks very unlikely if you look at comparisons like Iron Man, which had better legs and a stronger weekend, even if it replicates Iron Man's numbers it only gets to $395M. I think the only way it can still get to $400M is if Disney gives it a push; a re-expansion later on in its run. It needs a The Avengers style re-expansion. The Avengers went up from 123 to 1,705 theaters in its 18th weekend, giving it a +1,557% weekend increase. That gave it an extra 3M+. Maybe if GotG2 gets close Disney could be tempted to do something like that. I wouldn't count on it though.