Just realised that I forgot to mention that the 458 I purchased was an almost brand new CZ with known feeding problems. The previous owner returned it and claimed warranty (money back). I shoot at the same club as the previous owner and got a strange reaction from him when he found out that I'd bought it (at a heavily discounted price and without warranty) - news travels fast in these circles.

The LHS feed ramp has had a large portion ground out of it to try and stop the rounds jumping up and to the right but the projectile is actually past the ramp when this happens so it doesn't do anything.

Most of the rounds appear to be contacting the lug raceways even if they don't actually jam the action, as evidenced by the damage to the (wide) lead tips of the Winchester projectiles.