Melt Flow Indes (MFI), in its broadest, is a measure of "viscosity" of polymer material. Higher MFI indicates higher "viscosity" which is indirectly measured in specific gravity ratio. Higher MFI also means higher melt temperature and higher pressure to fill.
Flow path is the distance between tip of sprue entry point to farthest point of the mold component that is to be produced. Thickness is arithmetic average thickness of a component, if it has multiple thicknesses spread over. Higher ratio indicates that material has to "travel" longer to fill.
So, a material with higher MFI does not usually have higher flow paths although with technological advancements today, it is quite possible.
Higher MFI and / or Higher Flow path requires higher injection pressure and speed. So, usually accumulators are used to assit in such moldings.
Example: PC glass filled will have higher MFI than PC unfilled which in turn will have higher MFI than PP and so on. Basically, all so called "commercial" thermoplastics will have less MFI compared to "engineering" thermoplastics. http://en.wikipedia.org/wiki/Melt_flow_index
You can get this and more information in book, "A Guide to Injection Molding of Plastics" .
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