Matt, keep in mind you should let the stop watch be the final indicator as to gearing. But a Rotax senior should easily rev to over 13,000 rpm. A well sorted senior should be capable of revving to close to 14,000. Although that might not be necessary depending on the track.
We ran our MiniMax between 12000 and 12500 RPM last year without any issues and producing good power. We had more issues with heat and loss of power then RPM drop-off.