Being around in the 80's we drilled them...no chronos just the "thats better through the bit of wood" scientific approach.
I think we did see improvements up to a point but I really don't recommend this type of Tinkering.

Leave them alone or try to limit it to 3.5mm unless you can make plugs and revert if required.

I can't deny its addictive to tinker with springers though.