you can apply shaders within the editor (rightclick on the entity) or inside the fpe/fps-file of the entity (entitybank/segmentbank-folder) to change permanently.
HUDS will always appear stretched/rescaled from 1024x768, except v119-armorhud/radar/compass wich are not scaled.
to get the enviroment and objects HD, you need a HD-texture wich will cause more calculation and therewith more lag. in using more grungy material-textures, you can hide lower texture-solution easily, also you should use textures depending on the playerspots in the level (player stays far away > low texture, player can get near > higher texture) and also depends simply on the size of the object (doesnt make sense to give HD-texture to a pen).