7.5 hp = 5.6 kW - output power
phase voltage = 415/√3 = 239.6 V
239.6 (V) x 0.8 (A) x 0.8 (pf) x 3 (phases) = 0.46 kw - input power
The input power cannot be less than the output, so I think there is something wrong with your values. Once you sort this out you can get the efficiency be dividing the output power by the input power.
For the torque, there is a calculator on the site, which can help you (around 35 Nm for your example):