I have an 04 300(heavy flywheel) with a easy fitting Clarke 3.2 gal, and I have the RB carb and head mods.
Moderate tail speeds 5 and 6th gear 30-45mph, not in the power band. I have gone 85 miles with maybe a cup of fuel left. I filled the tank to the limit, and I'm not a heavy handed rider.
With you across the pond shipping would kill ya for the head and carb mod.
It made the bike more rideable, it lugs nice in the taller gears, and will still rev when you need it too.