I prepped using LPJ for about a month and found it a tad bit different at the actual advanced COMPASS test.
It definitely helped me prepare but some layouts were different. For e.g. the short term memory test layout on LPJ is a FD with 3 boxes on top labelled ALT HDG RADIO and the SPD box on the side. In the actual test, you just get the info and have to fill it out in the bottom. Not a cause of concern as long as your techniques are correct. Similarly, the CCT or Complex Control Task was slightly different but LPJ gives you a good practice, make sure you use a CH Products joystick and rudder pedals to get a better feel of it.
I would say invest in a LPJ gouge but go with an open mind, don't expect the same layout you saw on LPJ and you should be good.
Best of luck.