I can't comment on the lead sled, as I have never used one.

However, I would "think" that it's repeatability, if anchored securely, would deem better results than most "humans".

As far as you shooting low & left ALONG with "consistent" groups, tells me you may shoulder and cheek your gun differently than the lead sled is set-up to point towards...

I have one of my friends that shoots differently behind the same gun that 3 others have tried. He gets different results than us.
He does shoot HIS own personal gun fine, just holds it differently...

The key is consistency, so I wouldn't worry about the lead sled as long as your comfortable. An example, is if you have ever watched golf and seen Jim Furyk swing a club, I think it's God aweful compared to Tiger or Freddy Couples, etc... But Jim Furyk is a world class golfer, so "proper" form isn't always needed. But consistency is!!!