As stated above, the Vmo is variable dependent on altitude.
From the AOM.
0 - 8000ft : 245KIAS
to 10000ft : 282KIAS
to 18000ft : 286KIAS
to 20000ft : 275KIAS
to 24000ft : 248KIAS
For the best TAS (~365kts), fly somewhere between 18-20000ft. Not very flight efficient through.