Max speed allows you to safely u-turn out of latias/Latios which is huge
You just have to rely on your predictions. Low HP means you'll not take down more than 2 mons.
Also, Overheat is an option on Swords Dance Sets, to OHKO Ferrothorn (The OHKO is avoided if the Ferrothorn has full investment in SpD and has a SpD-boosting nature, but that's really rare, almost only theoretical). You don't get the nasty Recoil + Rocky Helmet + Iron Barbs damage and can take on even more mons.
Talonflame vs Ferrothorn
0 SpA Talonflame Overheat vs.
252 HP / 252+ SpD Ferrothorn: 288-340 (81.8 - 96.5%) -- guaranteed
2HKO
0 SpA Talonflame Overheat vs.
252 HP / 252 SpD Ferrothorn: 312-372 (88.6 - 105.6%) -- 31.3% chance to
OHKO
0 SpA Talonflame Overheat vs.
252 HP / 0 SpD Ferrothorn: 384-456 (109 - 129.5%) -- guaranteed
OHKO
But you have to predict right, as most Ferrothorns switch into a Brave Bird to rake up the said damage.
Overheat also KOs Aegislash on Blade Formé and it's especially useful because you don't have to worry about King's Shield Attack drop. Here are some calculations:
Talonflame vs Aegislash
0 SpA Talonflame Overheat vs.
252 HP / 252+ SpD Aegislash-Shield: 116-140 (
35.8 - 43.2%)
-2 0 SpA Talonflame Overheat vs.
252 HP / 252+ SpD Aegislash-Blade: 120-144 (
37 - 44.4%)
So you have no scope of KOing a Specially Defensive Aegislash if you use Overheat when it's in Shield Formé. But here's the calculation for the 252 HP / 4 SpD variant, which is really the most commonly used Aegislash spread:
0 SpA Talonflame Overheat vs.
252 HP / 4 SpD Aegislash-Shield: 152-182 (
46.9 - 56.1%)
-2 0 SpA Talonflame Overheat vs.
252 HP / 4 SpD Aegislash-Blade: 188-224 (
58 - 69.1%)
Rest assured, you can always OHKO an Aegislash in Blade Forme with the usual 252 HP / 4 SpD variant.