Can you please check as much of the cases specified below, and report your performance:
1a) vanilla 1.8 (that exact version, not 1.8.x), smooth lighting: maximum
1b) same, smooth lighting: off
2a) forge 1.8 pre-1523 build, smooth lighting: maximum
2b) same, smooth lighting: off
3a) forge post-1523 build, smooth lighting: maximum
3b) same, smooth lighting: off
4a) forge post-1523 build with forge rendering pipeline turned off (can be found under Minecraft Forge general mod settings in the main menu), smooth lighting: maximum
4b) same, smooth lighting: off