Are big-and-tall gaming chairs worth it if I'm under 6 feet?

Generally no. The big-and-tall variants in this category (Secretlab Titan Evo XL, GTPLAYER 400lbs, Dowinx Big and Tall) are sized for users above roughly 6'0" or above ~250 lbs. Smaller users frequently report that the seat depth is too long, causing the backrest to be unreachable without losing lumbar contact. Stick with the regular or small size if you're average height.