The Phantom Steeds can have a speed of up to 20, and the Shadow Walk ritual multiplies overland travel speed by 5. This means a party of adventurers can potentially travel overland at a speed of up to 100.
Is there any reason why this won't work?
I can suppose only 2 reasons why not, in other case it's DM decision agree this or no.
1) Shadow Walk ritual says: "You and your allies walk through shadows..." I'm not sure that magic created mounts can count as allies...
2) Phantom Steeds ritual says: " A phantom steed is immune to any effect other than damage" Positive (like +AC bonuses or other) or negative (like decrease resistance e.t.c) have no effect on Phantom Steeds, so Shadow Walk also have no effect.
*But your character can run faster than Phantom steed, so it's reason why ritual level is higher.