But the 350s have 28mm inlet port, for that a 28mm carb would be the best fit right?
-Sanket
Yes, if the port entry is 28mm, then the 28mm carb would be suitable. However, it is likely that any improvements would only be felt at higher rpms, compared to a 26mm carb. So, if you plan to use the bike at rpms around 5000rpm or even more, then the 28mm carb will help when the throttle is wide open.
There will be no difference at part-throttle conditions, compared to the smaller carb, because the throttle slide is presenting the restriction, not the carb size.