Sorry, I don't have any photos here at work, and I forget the measurement, but my subjective view is that it makes a huge difference. A torch that was small before is now really pocket sized.
I would not carry the FFv2 before, and if I was committing to a 1x123 light, it would be the Arc4. Last week I was away for a week, and I forgot my e2e/modded KL1 in another bag, but I made sure to bring the FFv2/CR2 because: it's real small (the scale of the KI, but cheaper and therefore less painful to lose), has two stages (I have a 20ohm resistor) and bright. It was always within arms reach of me, ready for a blackout in a highrise, or at the convention center, or just to look behind the headboard when my glasses fell behind the bed.
Much happier with the CR2 than the standard 123 tail. It literally gave new life to a light that just stood around and looked pretty.
daloosh