Just for a laugh I thought I'd give the HD texture pack a go on my 3GB 780s. Surprisingly it was playable, kept a high frame rate (70-90fps) 98% of the time but I had some quite severe frame time spikes,
I don't know if they were working properly, but to enable them I:
1) Downloaded the HD pack
2) Loaded the game, flicked the textures to 'Ultra'
3) Loaded the game itself
4) Restarted the game, not changing any settings to ensure things were properly reloaded with the 'Ultra' textures
Then to go back to High I:
1) Changed the setting back to 'high'
2) Quit to desktop
3) Reload into the game
Maybe it just refuses to load them if the you don't meet the memory requirements? But I couldn't explain why I was getting frame times spikes with them enabled but not on high (I made up a benchmark run to keep it consistent, going to a spot where the frame times spiked reliably, fighting the same group of enemies, running the same path, etc).