you can lower the compression with bigger head gaskets, you cant raise it as the compression figures for the pistons are based off 1-1.2mm head gaskets and you wont get one thin enough to make a big difference without running into issues with the pistons hitting the head.
idealy on E85 you want to try and aim high as your builder has said 9.5-10:1 if you can (arias make pistons off the shelf for the RB30 to do 9.8:1)