Tried to think of slow pro races in recent memory, to see what kind of kicks top athletes have off of slow pace to compare to Mu's 30s close to a 4:17 equivalent. 4:00-4:05 pros can close in 29-32 in a 4:10 race as a rust buster.
Best comparison off the top of my head would be 2021 Trials of Miles in Houston as it was an early, rust buster kinda race (February) that went out slow but closed fast, featuring solid domestic US talent and an Olympic top 5. Field including G DeBues-Stafford, Corey McGee, Shannon Osika, Sinclaire Johnson, Nikki Hiltz. GDS closed in a 29 last 200 to win in 4:10. Johnson and Osika behind her closed in 30 mid, McGee in a 31 mid.
In other words, they were able to close just as fast (or faster in GDS' case) while jockeying for position in a faster paced race. At the time, none of those athletes looked super sharp. Johnson didn't break 4:05 all 2021 for example.
I think Mu could go sub 4:10 in an even race right now, but even the gap from 4:10 to 4:05 is a chasm.
Re: OP's bold claim about Mu's global champs potential/beating the US 1500m record potential, let's look again at the winner of that Texas meet. Stafford front ran the heats in Tokyo, winning heat 1 in 4:03.70. Her last 400m was a 59.1, 44.0 300m, and a 29.3 last 200m. In the final she was 5th, a good ways back from 3rd and was no way near threatening Houlihan's North American record. Put another way, the gap in the 1500/mile between Mu and a great but not medal tier 1500m runner like Stafford is huge, and yet there is still a gap between Stafford and 3:54 kind of shape.
The way better, and more intriguing question to me, is could Mu win triple gold - 400, 800, and 4x800?