To every opinion, there is its opposite! I have to say that I think this is definitely a minority position, however. I walk to Finisterre/Muxia every chance I get. I don't ever carry more water/food than on my camino, but it is true that there are some stretches of maybe 10 km without services. But it's really nothing exaggerated. Highlights are many -- the lovely village of Ponte Maceira, the old stone albergue of Olveiroa and the As Pias meson for a great meal, the Dumbria albergue, the walk out to the beach at Lires, coming to the end of the headlands and looking out over the ocean before Cee. Just depends on whether you walk first to Finisterre or to Muxia, but either way, I think many people find it to be a very enjoyable walk.
So back to the OP -- 9 days for 250 km gives you an average under 30 km a day. Though I would say that's a rate that is higher than the average walker, it isn't extreme. I have walked many times to Finisterre or Muxia in 3 days and think that it's totally doable for someone in good shape, no need to be a star athlete to do that. It is true as MT says that most people walking out to the coast from Santiago have been walking a lot longer than you will have been, but I think that especially if you make your first day out of Santiago to Vilaserio instead of Negreira, the next two days are much more manageable.
Buen camino, Laurie