As there are limited information available I can only make a qualified guess based on the information given.

By changing the dip tubes to shorter size the pressure drop may have decreased however to the detrement of the seperation efficiency. Shorter dip tubes will cause an increased internal dust circuit of very fine (less then 10 micron) and hot particles from the lower stages to the upper stages.

This applies specificaly for the top stage cyclone were the factor of dip tube length to inlet height is approx. 1,2 to design for max. seperation efficiency.

However a proper preheater investigation should be done to find out any shortcoming prior to start modifications.

Best regards, Wizi