Let me give this thread a necessary update for future readers. I'm playing the Tolkiencraft 2 modpack, and had this same issue. However, I couldn't find the Player.dat file, nor any TC data records in the level.dat file. Instead, I found the Thaumcraft research data is now saved in its own file: <FTB install folder>/<modpack>/minecraft/saves/<world name>/playerdata/..
Delete both the <playername>.thaum and the <playername>.thaumback, reload your savegame normally et voila; research has been reset.