Friday you should be good, you have to get parking lot vouchers the night before or go on a standby list the day (on saturday and sunday)f, hard to judge how busy it will be and if this will be an issue or not. IF you are dune ready with your truck and take the truck and banshee out in dunes no need for the voucher and you can get in whenever you want. Just be sure to air down your truck tires and get all your stickers at the voucher center.
Its a crap shoot on the loudness, I have seen people that are good with just sparkies and I have seen someone get turned away too. I do know they sell the hush kit up there just as a worst case. I know of one instance on a VW rail where the person at the check-in stand told the driver to go and add more disc to their super-trap end so it will pass the noise level, if you actually understand how these work you will realize how dumb this is, anyways that person did that and was not made to run a sound check the rest of the weekend. I think you will be fine with just sparkies.