Aslain,
Fantastic work on your mod-pack. I'm reporting not a bug in the mod-pack but it might be left over in settings people have used from 9.3 and your pack:
If in prior packs you chose server reticule, then in the game settings (first tab) the option "enable server reticule" will have been checked. This seems to cause huge problems in aiming as the reticule jumps about and aiming is almost impossible. If you uncheck that then all returns to stability.
I noticed you'd disabled that option in the pack and wanted to let you and your modpack users know they should disable the option to have their aiming back.
Keep up the good work, it's always appreciated.
Calix