I wouldn't recommend Sprite Lamp for pixel art. If you're going to hand pixel things, hand pixel the normal map, that's my idea. Sprite Lamp is not a new idea, it just makes it easier, but also sloppier (as it's auto-generated and not hand pixeled).
Also, about purism: normal mapping can be done (and has been done) on very old machines through palette cycling, there's nothing "faux" about it. We just now have the power to do it in a much more dynamic fashion.
As for the original question, I think I agree that it's probably not going to look good to increase just the framerate for 2.5D. The question is: where does the uncanny value start? From experience, I'm thinking for sprites that big, you CAN go up to 8, but that's pushing it just a little, 6 for a walk would probably be what I go for.