I have never experienced these issues with vanilla, but to this day with every mod compilation I made for myself or modpack, so my guess is that mods do cause more lag than vanilla?
I have no idea about percentage, but my guess would be about every other player, so near your percentage.
To this day and in every version I have played modpacks including FTB and DW20 optifine has been the solution for framerate issues for me, so yes, another mod has been fixing the problem.
And I think your reasoning to adding optifine is a bad idea (each on his own, not in the pack) is just wrong, I wonder how my clients would react if I would suggest that it's not a good idea to modify their server to run faster, but just reduce the number of users, would be fun times.