That's ok, but I wonder why you'd bother with a sunder attempt and risk an AoO (possibly from the rider, too) if you can just hit the thing?
Having it apply to regular non-Sunder fiery weapon attacks just makes it too easy to bring them down.
I'm sure there are going to be circumstances when you want to take a dragonfly turtle out of the air and only have a torch or
flaming weapon, but are prepared to risk an AoO.
They're a lot easier to deal with on the ground - no diving attacks and it's easy to play "keep away" and kill them from a distance.
Besides, a flask of alchemical fire is a cheap and easy way to bring them down.
I'd rather keep the Sunder.
We could add that the wing-burning also occurs on a critical with a fiery weapons.
e.g.:
Vulnerable Wings (Ex): A dragonfly turtle's wings are highly flammable. They can be damaged by area-of-effect fire attacks, or by a critical hit or Sunder attack from a flaming weapon (The wings have AC17 against Sunder attacks, and sunder damage to a wing does not harm the turtle). The dragonfly turtle must succeed on a Fortitude save (DC equal to 10 plus the fire damage taken) or lose wings (one wing per 4 points of fire damage). A dragonfly turtle which has lost a single wing cannot fly, but can land without taking falling damage as per the
feather fall spell. A dragonfly turtle that has lost two or more wings takes normal falling damage.
Additionally, dragonfly turtles cannot fly with wet wings. If a dragonfly turtle is struck by a water based attack or effect while flying, it must succeed at a Fortitude save (DC equal to that of the attack or effect, DC 15 if not otherwise specified) or be forced to land as if it had lost one wing. A dragonfly turtle can dry wet wings by buzzing them for at least one minute, or longer depending on the circumstances.