It seems to me that once you're in the terrain of "feeling" requiring a (suitable) nervous system, you've already crossed the borders of "p-zombie" experience, having already recognized analogous behavioural and physiological reactions as evidence of "feeling" (whatever that might imply).

I get that your point is much broader; the meaning of "feeling" is going to keep staring you in the face when thinking about a question like this, but it's a sophomoric question to anyone who accepts the possibility of it's discovery without a metaphysical understanding of a sunflower's conscious experience.