If you slow the engine to 1/2 speed, you will get approximately 1/2 the power output. 7.5 kW is a bit on eh heavy side for a 12/2. At 325 RPM, you could figure on no more than 3 kW, except for surge loads. Chances are that the SFC (Specific Fuel Consumption) will be higher, because of less effective atomization of fuel by the injectors.

What can you do to re-balance or reduce vibration? Can you install a better muffler system? Do you need 7.5 kW of generating capacity? If so, the 12/2 will be hard pressed to do it on a continuous basis.