Heavier arrows increase penetration every time vs lighter arrows. More penetration cuts up more lethal tissue than less penetration, AND leaves a better blood trail w/ and exit wound. Guys who say this and that probably haven't studied it much. Read up on Dr. Ashby's studies on penetration. He's spent years laying scientific trials and analysis to his arrow penetration studies. Be forewarned, he goes at it from a scientific viewpoint, and there is TONS of his data and analysis out there - youtube and written format all over the net. It can take days to go thru everything he's done on the subject, and digest it. Most guys won't want to bother themselves with true details only to arrive at an answer that doesn't meet their latest acquisition of whack 'em and stack 'em equipment.
Basically, he is a heavy arrow fan, with cut-on-contact heads, and a big fan of extreme forward-of-center balance on the arrow.....AND true arrow to bow tuning. If an arrow/head combo isn't perfectly tuned to your bow, your style of shooting, your draw length, etc., it won't do as well as it could on penetration. Paper tuning is the only way to go on true tuning IMHO. Bare shaft is probably best to start. Ashby has found that a minimum total arrow weight of 650 grains or more increases penetration very noticeably and consistently, and says his data shows that to be true from at least 40# draw weight on up to at least 100#, and in stickbows, compounds, whatever. Speed is much less relevant than arrow weight on penetration.
Read up on Ashby's stuff if you haven't - it's very interesting, and nobody's disputing it after years.