Do you have pics of your replaced headlight lenses?It is unlikely the projector lens itself has yellowed. Some projector lenses use a "fresnel-type" of shape that slightly softens the projector's light cutoff. Rather than spend hours trying to clean and buff your lenses, you can replace them for $25 with perfectly clear replacement lenses from these guys. 80% of the effort is related to R & R the headlight and bumper, 18% goes toward baking the headlights to R & R the outer plastic cover.
You will need to bake the whole light housing in the oven to gain access to the lens. Although you can remove the projector from the back, without baking, you can not remove the shroud around the lens which is a chromed plastic donut that fits around the projector lens.