They don't play well with CURRENT sabots because 1) the sabots are too short for the longer bullets and 2) currently you are correct that the .451+ bullets would be to heavy and the needed .400 bullets need such a thick sabot that it doesn't release from the bullet. BUT here's where the fun begins. With the lead free designs now out there, that means you can have more length with less weight! Therefor you can now have 250 to 300gr bullets in .451 with a more efficient BC profile and length
. TADAAAAAA