Vingegaard did 6.2 w/kg in the last 33 minutes of La loze, and the conditions of that perfomance were worse than the conditions of tomorrow, it's a unipuerto stage.Tourmalet from Gripp (top with 47 km to go in the Tour last year) has similar altitude, but is a bit longer. Tourmalet (2115 m), last 12.37 km at 8.83 %, Vingegaard and Pogi: 36'54", VAM: 1775 m/h, 6.16 W/kg [Dr.F].
If it rains tomorrow, they won't go sub-30'. ~29'30" should be doable in good conditions.
La loze was more long, after 16 days and a hard TT, high altitude than Vallter 2000 (2350 m, Vallter 2150), and the stage was more harder.
I don't think 6.4 w/kg is difficult to achieve.