Yes it's a good idea to discuss all limits, soft and hard before any scene. I make it clear what limits will get tossed right out the window depending on my mood. Yes, subspace does make things hazy. My pain tolerance is much higher for one, so I tend to lose my ability to say 'when'. I however am fairly certain that if something were to press too hard on my hard limits, I'd snap out of subspace, and probably be pissed about it.

I have few limits at all really, but I do have some. Of the few hard limits I have... I really really mean it.

No, I don't think it's fair to 'take advantage', but if you have open and clear communication, there is no fear of that happening. You as the Dom know what your sub is willing to do. Only you can know them well enough to know what they will be comfortable with after the deed is done. Sit down, and have a chat with your sub. Limits get pushed, slowly. The chats need to happen often as things change.