OP:
I'm not sure, but I think functionalists like to explain suffering by identifying it with pain and say that it's some kind of detector that makes us avoid the things that hurt us. If suffering is something like that, then it seems like we have to say that any intelligent system with a similar sort of mechanism would have the capacity to suffer. But imagine a vacuum cleaner robot equipped with a heat-cencor receiving the signal 'turn 5 feet back' once it registers a certain temperature. I know it's a simple case, but would you say that the vacuum cleaner experienced pain when it turned 5 feet back? Why, or why not?
So the answers to your first two questions I think depends on your notion of pain. If you deny that it's anything other than physical/functional processes that's involved, as we all seem to do these days, then you would be hard pressed to show that pain in some system differ from another in terms of 'realness'. (I don't think you could that in any case, actually, for what would make some pain, once recognised as such, more real than another? Certainly not how much it hurts?) And given this, there definitely seems to be some bias in that there is still some resistance in acknowledging other systems (ie. robots) as potentially as real as us.
At the same time I'm getting tired of the trend towards (can I say this?) belittling human beings by constantly reducing our behaviour, feelings and thoughts to biology/neurophysiology. For example, the idea that art is a strategy for reproduction, or talking about happiness as the release of serotonin etc. I'm sceptical that this is the most instructive/beneficial mode to understand and relate to happiness, art, or pain in our own lives. Telling ourselves stories about our moments of happiness, or why and how we choose to do art, I think, often gets us closer to what happiness and art means, and subsequently, in a way, to what it is. I guess my criticism of reductionism would be it's narrow way (only looking at the efficient cause) to go about explaining/understanding pain, and it's dismissive attitude towards other ways of explaining/understanding pain.
For say, left out of this picture, and one very good reason why at least I would hesitate to compare our pain with the pain of a robot, is all that is bound up with our pain. It's something we all share in, and know that we all share in. In other words, we have feelings and thoughts about it which sometimes leads to a piece of writing or music or whatever else. It leads us to do all kinds of strange things, like Dostoevsky can write about this russian woman that fell in love with someone that she could have married anytime she pleased, only that she constantly invented reasons for herself for why she couldn't and, owing to a particularly picturesque landscape that day, ended her life throwing herself into a river reenacting Shakespeare's Ophelia. No biologist or neuroscientist, not now, perhaps not ever, could shed any meaningful light on such a case, and neither on what it truly feels like to be in pain. For such cases we need other modes and I don't want to see them marginalised by reductionism, or by some idealistic scientist who finally believes to have shed his last piece of humanity in pursuit of some specific kind of knowledge. Pain, and I would really like to call it suffering now, is so bound up with all that we are that to isolate it and speak of pain in the reductionistic mode is to miss most of what is interesting and meaningful about it.
Oh, and I'm not sure if I can agree to your other question, because I don't think it's the case that "phenomenological 'inner worlds'" are postulated at all, except perhaps in the past tense.