I usually find with hoods it's not pipe work that get clogs it's the grease filter which are doing what there meant to as grease in pipe work can become a fire risk, plus fan don't like it either. Better to have a cleanable element to them.
No efficiency gain possible in my view. But the original extraction rate can be improved particularly if it's a long run to the outlet. With a specialist designer they'll look at (in no particular order) the extraction rate, noise, proximity to cooking fumes, air flow in kitchen, filtering, flue design, fire safety, building regs ++.Do you think getting a regular extractor fan and putting an in line mixed flow fan will double or triple the extraction rate? Hence becoming a lot more efficient?