MAGE - Mildly Advanced Graphical Enhancements - is a mod devoted specifically to extending the graphical customizability and capablities of Minecraft. It is, as such, client-side-only.

Please note that the mod is in a very early stage of development and that the following list of features and supported versions is subject to growth, change, etc.

Features

• (0.2.0+) Smooth water - fixes smooth lighting issues inside liquids and on the surface of water. Also its own mod.
• (0.2.0+, WIP) Partial support for MCPatcher's resource pack "Custom Colors" functionality.
• MCPatcher-compatible custom lightmaps; allows you to use resource packs such as "Pixel Reality: Luminance" without OptiFine.
• Optional "true darkness" tweak - configurable, works with custom lightmaps and non-vanilla worlds.
• For developers: most of the hooks the mod uses for graphics tweaks are exposed as an API, which means you're free to provide custom implementations as part of your mod.

Supported MCPatcher features:

• Custom Colors (potions, text, redstone, pumpkin/melon stems, pine/birch)
• Custom Lightmaps

As I understand, the mod does not impact client-side performance visibly (and any future performance-sensitive functionality will become configurable). I think it is fine to include in a modpack by default.