Okay, I found out how this worked. Basically, much like how a Breaker skill against Wary Fighter cancel each other out and it than goes back to mattering about Speed on if a Double happens or not, basically Siegmund's new effect + Windsweep's no Follow-up effect cancel each other out, thus when it comes to doubling, it now relies on Ephraim's speed on if he doubles or not. If he has 5+ Speed over the enemy's Speed, he can double. Otherwise, he doesn't double.
And by what I heard, he actually can reach good Speed numbers with the right setup. Basically, if he's a +Spd, he can reach 29 Speed rather than 25 when a normal IV. Than you give him either Fury or Life & Death which gives him an additional 3/5 Speed, making it 32/34. Than you give him a SPD Seal to get to 35/37. Supporting him will make it 37/39 and 39/41 when paired with another ally. Have that ally be Brave Lucina, with her Prf Weapon, Drive Speed and Hone Speed Seal, which gives him an additional 10 Speed, making it 49/51 Speed altogether depending on whether you chose Fury or Life & Death.
I don't have a Spare Nephenee and you'd be crazy if you think I'm going to SI my Free Summon +Spd/-Res Nephenee who helped me so much during the Black Knight Tempest Trial.
I do have a Spare Cecilia and Lon'qu though, hence why I'm asking which one should I do? Escape Route or Vantage?