Puting it simply, there's no way you will get 5 amps out of this without also releasing the smoke genie
If you want 5 amps at 8 volts then you need 40 volt-amp
If you need 5 amps at 15 volts then you need 75 volt-amps
simples
The reason that ac power units are specified as volt-amps is because the supplier has no way of knowing what power factor your load will be. At unity PF volt-amps is equal to watts. However at reducing power factor the available watts is also reduced since watts is specified at the equivalent unity power factor.
Hence if the unit is specified as 40 volt amps with an 8 volt output then you will always get 5 amps out of it, even if you have a low power factor (inductive or capacitive load) only represent say 20 watts
jack