Comparisons and conclusion

Work

Incline: Work=322.923J Power= 645.846 watts
Decline: Work= 374.678J Power=1023.710 Watts
Flat: Work=370.0585J  Power=652.660 Watts


Comparing all three, it is pretty clear which one generated the most power.  Decline by far generated the most power, it had the greatest acceleration and since the distance wasn't that large, the time it took to the complete the lift was relatively short.  Incline and flat were nearly identical in power although incline required less work.  


Conclusion


In a physical sense, there really isn't an accurate way to incorporate the angles of the bench and how it affects the actual lift.  The lift itself is determined by a simple up and down motion and what affects the work and power in a physical sense is simply how fast you are able to move the mass and the total distance it takes.  The issue with looking at this problem at a physical sense is that when you try to incorporate the angles, the forces are already balanced and the press doesn't utilize the angles in that sense since it is a straight up and down movement.  

The image above does a good job representing the issue.  The top box can be seen as the forces acting on the barbell.  The box on the incline represents the person on the bench at an angle.  As you can clearly see, both are independent of each other in the forces sense.