The "Made in Japan" Sony AccuCores I'm currently using have media ID YUDEN000T02.
However if your AccuCore package says "Made in Taiwan," chances are they're not Taiyo Yuden,
but still 1st class if they're one of the SONY media IDs.
You should definitely check for updated firmware for your drive. Older firmwares simply don't know about the newer media IDs and default to slow speed with media they don't recognize.