Just try a 280 then. Before removing the old shock, measure the rear height of the bike with you sitting on it. Put a new shock in then reset the rear height via the spring preload. If the bike does not turn into corners easily, then the rear is still too low (ie. forks are choppered out). If it's unstable in a straight line then the rear is too high - which means the stock shock length was all wrong. If you cannot drop the fork clamps a little (eg. front tire hits the bashplate), then instead maybe extend the ends of the swing arm. That'd raise the rear a little and who knows, the longer wheelbase might help the handling. I run the axle full aft on the MSO with 14" rear and it handles heaps better.
In other words, I'd not assume the stock setup of the bike is right. Just try things, and who might be able to improve its handling.