What is the impact of High Exhaust Pressure on HRSG?

Higher Back pressure means the HRSG offers resistance to the flow of flue gas.
i think this should be around some optimum value as
too high resistance would mean not enough flow
too less resistance would mean the flue gas moves out too easily leading to an inefficient heat transfer
Muhamad Adeel...
Once I worked in a plant where an HRSG was referred to as "Flue-Gas-Cooler". The company was heavily fined when the country's Authorities in which it was located ruled that it was a Heat-Recovery-Steam-Generator and not a Flue-Gas-Cooler.
Regards, Phil Corso