Yeah, many of the drills are massively harder in speedskates, especially the really long 5x84mm frames (340mm) and doing crossover crazy eights. That's what tires my ankles out much more than the low cut boots.
Still, your weight and edge control is so good you shouldn't suffer nearly as much as the rest of us. Then again, if you have low cuff boots you'll take a fair bit of strain because of skating in FSKs so much. FSKs have so much ankle support because of the jumping they're designed for.
Didn't Alan buy Rick's 5x80mm frames? Maybe you could borrow them. You can also borrow my RB lightning 05s if you prefer those, the frame is 4 wheel but is 285mm, I guess you may as well use FSKs as they aren't that much longer.