I took a look at chapter 17-4 and already was seeing enemies appear that a max-level Mordi couldn't double. That's not to mention the double-standard this means in regards to Ilyana (Mordi's SPD is fine enough for him to get a 8/10, but Ilyana, who can get more SPD than him, seems to get benched over speed problems).
I found one enemy:
1x Pegagus Kn lv 17 (iron lance)
25 hp, 19 atk, 17 AS, 113 hit, 39 avo, 8 def, 10 res, 7 crit, 5 cev
Which would be OHKOed regardless.
So what I'm going to do is actually sit and name off every enemy that has 16 or greater Spd from 17-4 - 28 (because nobody gives two flying fucks about Endgame) and we're disregarding Myrmidons / Swordmasters because they obviously are too fast..
18: Okay, so I can admit the Ravens would be an issue. The boss has 15 AS, 2 Sword Knights and an Archer have 14, and then a Fighter has... 12. Everything else is roughly 10-0 AS (yes, some enemies have 0 AS).
19: Excluding Ravens and Naesala, the boss has 20 AS. The next highest? A Bow Knight with 12. Everyone else averages about 12-10 AS.
20: Shiharam has 14. Everyone else has like 12 or lower.
21: Ena has 18, the miniboss has 15. A Sniper comes in with 14 AS and a Halberdier comes with 13. Everything else is 12 or lower.
22: Schaeffer has 18 AS. The Thief is easy to OHKO (28 HP / Def total), a Tiger comes with 16 AS and a Cat comes with 15, and everyone else is 13 or lower.
23: Disregarding Myrmidons, enemies have 15 AS (note: only a small handful do) or below. Mordy should go nowhere near Petrine.
24: There's Rikard with 19 AS, a Berserker with 16, and a Paladin with 17. 2 Enemies have 15, Bandits can have 13, and the rest are 12 or lower.
25: Aside from Gromell:
1x Cat lv 7 (claw)
37 hp, 24 atk, 16 AS, 119 hit, 33 avo, 15 def, 7 res, 7 crit, 1 cev
3x Cat lv 9-10 (claw)
42 hp, 27 atk, 18 AS, 114 hit, 38 avo, 18 def, 9 res, 8 crit, 2 cev
1x Raven lv 11 (beak)
38 hp, 21 atk, 20 AS, 124 hit, 42 avo, 14 def, 11 res, 8 crit, 2 cev
1x Hawk lv 12 (beak)
42 hp, 25 atk, 19 AS, 129 hit, 41 avo, 16 def, 9 res, 9 crit, 3 cev
1x Tiger lv 14 (claw)
49 hp, 34 atk, 18 AS, 133 hit, 39 avo, 20 def, 9 res, 10 crit, 3 cev
Turn 3 Reinforcements:
1x Cat lv 7 (claw)
37 hp, 24 atk, 16 AS, 119 hit, 33 avo, 15 def, 7 res, 7 crit, 1 cev
1x Cat lv 10 (claw)
42 hp, 27 atk, 18 AS, 114 hit, 38 avo, 18 def, 9 res, 8 crit, 2 cev
1x Raven lv 12 (beak)
38 hp, 22 atk, 21 AS, 127 hit, 45 avo, 14 def, 11 res, 8 crit, 3 cev
The Snipers have 15 AS (4) and a Halberdier. This is one chapter where Mordecai could do kind of iffy at.
Chapter 25 and thereafter is the few times enemies are actually above 16 AS (well, a very very good portion of them). They usually add up to the boss (which obviously Mordecai can't tackle), the Myrmidons / Swordmasters (small handful), and maybe a couple other units depending on the situation.
Now, I'm not really saying Mordecai is so goddamn awesome (quite the contrary, I don't really like Mordecai all that much), but to say that enemies are really that fast in this game is kind of a joke. Furthermore, we rag on Ilyana not really because of her Spd. The big culprits come out to be her offense being a small redeeming point. Her Spd base isn't terrible, but her growth is pathetic (30%, 35% with a band) and a promotion throws a whopping two. Like Mordecai, she's likely going to slam one huge hit on an enemy and leave it at that. Ilyana also has shitty durability (20 HP | 3 Def is pretty amazing, right!) and shitty Movement (6 Mov woohoo!) to go along with her other issues.
There's also Smite, which once again is a big thing to talk about. Mordecai can fucking Shove / Smite
Jill untransformed, which is pretty powerful to have. Now your mounted units can have about 10 / 11 Mov instead of the usual 8 / 9. Mordecai also gains another 2 Mov transformed, which means 9 Mov bad ass shoving machine. Pity that he can't Shove Haar, but that's the only unit of the damn game.
Speedwings are a valuable resource never the less.
Oh really, well so is BEXP. Now can you name me what units seriously can use the Speedwings and not give a diminishing return, or are we going to point out the Captain Obvious?
I was considering that, in order to work with a SPD band on the field, he needs to equip it which will make him unequip the band (and thusly untransform) so he won't be able to get the kill to level-up.
A 5% boost makes negligible difference. It would take 10 levels just to make it +50% growth. You're not really speeding up his Speed growth very quickly with it.