My understanding of expansion chambers and silencers is that they are designed from the factory to give you an optimised spread of power over all the rev range. They are designed as a compromise .By altering the length and diametre of any of its components will affect how the engine runs.
Im pretty certain there was a thread on here last year were someone had dynoed there RGV with Standard cans, No cans and I think some aftermarket ones as well .These all had a dramatic affect on the way power came in. Without the cans on there is a lot less back pressure so the motor was peaking at around 7000rpm .
I suspect if your cans are from another model they will have been tuned in length and diameter to that engine so this is probably why it runs poorly.
BOSOZOKU.
_________________
MC21R Repsol replica
SC36 'Fire storm'
RG250 MK3 GAMMA 'WALTER WOLF'
DR350SEV winter hack