An hour or two in the FFS won't make you a polished performer but it will take away the surprises, calm you down, and allow you to fulfil your potential in the assessment.
Until you find that you aren't in an 800 FFS.
Don't waste your money: you will be briefed on the sim trip. You will not be expected to fly the sim to LPC standard. We all know that a guy coming from a PA34 will find it tricky.
Turn up, be yourself, fly the sim to the best of your ability.