A-Frames don't necessarily penetrate deeper than Partitions, even though they retain more weight. (At least in smaller calibers. Many of the bigger Partitions are designed to retain around 85-90% of their weight. But I don't know if 250-grain .35's aare designed that way today.)
A lot depends on velocity. A-Frames are actually softer than Partitions, despite the front core being bonded. They're made with super copper and, I believe, pure lead as well. Consequently at higher impact velocities they open up wider--and the rear core (which isn't bonded to the jacket) tends to shift forward, expanding the rear end as well. In this case the extra diameter tends to limit penetration.
A Partition is made with harder jacket and core alloys, with the exception of the front core, which is pretty soft. Because the front end isn't bonded, it tends to open up quickly (and violently) but then the jacket curls back over the rear of the bullet. This creates a smaller frontal area than with the A-Frame.
The rear end has a harder alloy and, in many cases, slightly thicker sidewalls of a tougher gilding-metal alloy. Thus the rear end doesn't usually expand as it does on an A-Frame--or at least not as easily.
The end result is that penetration is usually similar with thw two bullets, though much depends on impact velocity.
In the instance you're asking about, you're not going to get much more than 2600 fps out of the 280 A-Frame. Or at least that's what I get out them in my own .358 Norma. This isn't really high velocity, so the bullets won't expand as much as they would if going faster.
In my own rifle the 250 Partition outshoots the 280 A-Frame by a noticeable margin, but that doesn't mean it will in all rifles. In a .35-caliber rifle accuracy would also tend to involve the rifling twist. Some still have 1-16 twists and might not stabilize the 280 well. I believe that's the case with my own rifle. But newer 35's tend to have tighter twists of 1-14 or even 1-12 and will shoot any bullet weight quite accurately.
