Most of the subsonic loads I've seen for the 458 are with 500gr to 600 gr bullets. I suspect those heavy bullet loads would have enough pressure to cycle the action and still be be subsonic before your modifications. I think trailboss would be a better powder, unique is a pretty quick burner.
Trail Boss on the Burn rate chart is actually a significantly faster burning powder than Unique, 23 vs 41. I don't think the burn rate is the actual issue, it is the amount of gas produced/volume of powder and there TB produces significantly more gas volume because it was designed as basically a Black Powder substitute for heavier weight bullets. Ten grains of Unique occupies about 20% of the volume 10 grains of TB will use. Also, TB throws great and accurate from powder throwers so makes high volume loading very easy.