Just adding my experience here.
I ran C4 heads in the 390 in my Highboy. C4 heads are known for the same issue with lack of support for hardened exhaust seats. Since it was going in a hard working Highboy, I had decided to use hardened seats before I knew of this issue. The very experienced gentleman who did the machine work thought it would be okay, and we're talking about at least 30+% of the seat support was gone.
I ran those heads for over 23 years in my Highboy, with lots of abuse. 2 years ago, I gave them a light valve lapping, and with no other work than a good cleaning (and a couple new valves from bad pitting), threw them on the 390 in my '65 Galaxie and drove to Atlanta and back, from NW Ohio. I have not seen a single issue with them. That engine regularly got spun to 5500, and occasionally to almost 6k. I'm convinced, with good installation, the lack of support is not an issue unless you're going to be thrashing the engine hard in a racing environment.
The Medium Riser heads in my Mach do not have hardened seats. You know what it's been through, thousands of street miles on Drag Week and at home, racing DW, a couple of other events and the FE reunion. I use low lead race fuel on the track, but street driving during DW always saw 93 Premium. I usually keep a mix in it at home, because I know I'll be having some fun with it anytime I drove it. I have yet to see an issue with the heads, with no hardened seats.
Blair used hardened seats on the C4 heads that I bought off of him and put in my F350, which pulls my heavy car trailer. He originally built the engine with a spirited cruiser in mind, and knew of the support issue. I think he doesn't have a problem with it either, assuming good installation.
It almost seems to me you could go either way and be fine, unless pulling heavy loads like Bill said. Then I'd definitely opt for the seats.
