Regarding gas turbine GE 6B, design exhaust energy is about 290 mmBTU/hr. If gas turbine degrade, exhaust temperature will rise & exhaust mass flow will drop. However, GT exhaust energy will drop. Could you please provide/approximate %exhaust energy degradation rate of GE 6B in term of %per year?
It's not a linear degradation over time. There are a LOT of variables which can affect the degradation, some of which can be mitigated, such as with on-line and off-line compressor water washing and changing turbine inlet air filters.
The packager of your GE-design Frame 6B heavy duty gas turbine probably can provide what are called performance correction curves which may be very helpful. You might even find them in the Operations & Maintenance Manuals provided with the unit.
You can also download a GE publication, GER-3620, which may help understand some performance considerations. It has information for a lot of Frame sizes which can be difficult to read through and find exactly what you're looking for, but it will probably be very helpful.
Hope this helps.