GE Fr9E Units Combined Cycle Operating with High Back Pressure

The site configuration is 03 Fr9001E Units + 03 HRSG + 01(common) Steam Turbine. The HRSGs were a poor design and resulted in the GTs running with continous high exhaust pressure ( 410 ~ 420 mm H2O, which is close/above the alarm limit), while the situation as expected gets worse during winters for 1~2 months ( 445 ~460 mm H2O). The trip is set at 520 mm H2O. The Vibration levels and axial thrust have remained within normal limits during this time and the exhaust section inspection did not reveal anything unusual. Going forward, there is a plan to install inlet chilling on the units which will result in constant very high back pressure similar to winter conditions. We are contacting the OEM, but will appreciate if someone can share similar experience and knowledge with regards to high back pressure and mechancial integrity of the units. Thanks.