Mine has a 10" speaker, which I imagine will already make an improvement over the 8".
It sure has pretty low headroom, but I've never used a "clean" amp sound as a base to build upon, always start with the amp breaking up a little anyway, so that isn't too big an issue for me...! I also have a 12AT7 (or is it an AY7? Need to check), which I bough a while ago as it should help a little in the way of at least apparently increase he headroom but as it were I never ended up trying it out...!
I've found a second hand g10 Greenback and will probably pick it up in the next few days, so might play around a little while the amp is open and try new speaker AND new valve to see what difference it makes...