A three-quarter mile workout

[Fazerider]

So are you saying you can gain about a litre of space then just by opening that little hole up?

Yes. (Unless you're patient enough to keep filling very slowly as the air bleeds though the tiny hole.)
I saved the job for when needed to clean the tank out anyway for a repair on a rust hole. I didn't want swarf clogging the filter and had concerns about the fire/explosion risk.
I guess braver people than me might figure a way to catch the filings with a magnet and do the cutting with a full tank... theory being that the greater the air space available the larger the potential bang if the concentration of petrol vapour gets low enough. For the same reason it'd be safer to do it on a hot day rather than a cold one.
I'm sure of the science, but am also a coward... so made certain the tank was vapour free instead. :lol