Without knowing other
critical performance properties it’s difficult to speculate on the effect of raising resin melt (higher MI). The usual result of raising MI in a GF filled PC is lower impact.
It might be possible to balance potential impact loss due to changing to a higher melt resin w/ slightly more GF or other formulation changes. Again, if the target properties are well defined, your compounder may be able to formulate a "way out" of the small processing window; smaller fiber length, alternative coupling agent, internal lubricants, PET or PBT alloying, etc.
I would not rule out a change in filler. As suggested earlier, the present grade may be over kill. A properly coupled (surface treated) mineral
filled PC may do the job; meeting impact requirements and other key properties, while offering a wider processing window. Your compounder should be able to help you make a better resin selection suitable to address both desired end properties and ease of processing.
BTW - I think diluting your present resin w/ a higher flow PC will usher in other problems associated w/ poor dispersion and poor distribution of fibers, probable loss of impact is only one of the potential problems.