I would love to know what differences you made in implementation to get down to 45C for 600mhz. That seems like a really drastic improvement.
I'm unsure that he mades so much improvement.
As I understood vivado reports on his design >12W which must be dissipationed against 400 MHz github design which reports <9 W and gives temps near by 60 °C.
But there chip to cooler heat conduction is possible better.