i'm a but confused with the "looking to build an ej207 with......sti semi-closed deck." do you mean the jdm ej207? i dunno. either way...
AVCS is extremely useful. you'll definitely want that with your build, especially if you are looking for 400+whp, because it does help with spool characteristics also. if your ecu and heads are setup for avcs, then the aftermarket cams that are made for it will use avcs. what kind of heads are you running? JDM v8 heads are an amazing set, otherwise v7 or any other avcs heads would be nice. valves, springs, and retainers are worth looking into if you are looking for serious power.
if you ae looking for a serious build, a stroker kit would produce much more power with the added cam upgrade. it does seem you give 2 shits about daily driving because of your idling statement, so getting to aggessive may not be for you. a built rj207 is capable of producing 400+whp. i personally like cp pistons and make sure you bore the cylinders out for better ring seating and piston fitment. i have to go drive my gf to her work so let me know if you want to continue talking,