The bearing surface of a bullet is what it is regardless of seating depth. That doesn't change.

Seating dept should be about the caliber of the bullet's bearing surface supported by the case neck. The boat tail will likely be well into the shoulder area (or slightly below). All you lose is power space - but longer bullets/heavier/more bearing surface need less powder to get to the maximum pressure, anyway. Some, like the .223 Rem, don't have enough neck but shoot well in spite of it.