That is why it must be made as a launch option. When modpack is stable and ready for deployment, all errors are resolved, etc. Creator of the pack just adds -nolog option or something like that and the pack will (maybe?) load faster. If something goes wrong, user can remove -nolog from launch options.
The title says it. This is just a suggestion - I think removing log calls while forge is loading is a good idea to increase loading speed. Heavily modded minecraft can generate insane amounts of log data in files and console while launching up - and every log line slows loading a tiny bit. I don't know can or can't it be done so this is just a suggestion.