True, a seat replacement will last longer than the original - which lasted 50,000 miles. If you expect to do another 50K, it's a good investment.
However: it's a precise operation. It's expensive because it's not easy work. New seat hole is not centered = port is ruined. Press fit is too loose = seat comes out. Press too is tight = head cracks (which may happen if overheated anyway). Been there, done that. The heat transfer from the insert to the casting is not as complete as the original. Even if the work is guaranteed, who pays you for R&R the head?
The change in chamber volume is far too small to have a significant effect.
My choice: a SBC 1.60" valve can be reduced to 1.53" or so (no larger than you need) on the re-facing machine, and permit the seat to be re-cut to the original depth and stem height at 1/3 of the cost.
