Among other things, the CRZ Mach No or Cost Index, will change your optimum level. Assuming these factors are constant, and only the performence is the criteria, then the optimum level is just that. You may however climb 2000 Ft above optimim, but you have to be aware that your buffet margin is now 1.3 g instead of 1.5 g. If no turbulence is forcasted, you should be ok.
One should avoid to climb above optimum level if shear ratio is on the high side, i.e turbulence forcasted.
I just hope I did not cause more confusion on the subject that is very little understood even among pros.
cheers.