Since they've actually added weight to the ETC501 rack now, theoretically it moves the center of gravity forward. I haven't flown it enough to figure out if it helps or not, but I have been taking the DT every time I go up (usually 75% and DT, drop the DT on climbout somewhere, or keep for really long sorties).
If I really want to mix it up I'll take 2 guns only. I fire my MG131s with the MG151/20s always (2x 50cals are enough to kill planes, these will too). I tend NOT to take the MG151/20 outboard guns, will more likely take 2 guns or outboard 30mms.
I find when I take 4x20mm, I get a lot of hits with less damage. I don't know why. Seems less effective than the basic 2-gun setup, even! So I'll take the 30mm if I'm "maybe fighting but really looking for big targets" and no outboards if I'm not.
You can tell when you land a 30mm on a fighter. Instant decapitation!