Actually the discarding of his spiritual pressure, as Aizen put it, fits in very well with Ichigo's bankai. His bankai is basically a method of increasing his physical prowess by forcing all that spiritual pressure into a smaller form than traditional bankais. Being able to use all of his spiritual pressure in this matter is a perfect extension of that. Hejust has all the more spiritual pressure and his efficency has gone up to 100%.

Also consider that, all other things aside, Aizen is a smart man. His analysis of Ichigo is likely accurate, at least as far as the spiritual pressure goes. I suspect Aizen will evolve again and Ichigo will pull out one more elevated form with which to slap Aizen back down.

I think that spiritual pressure far surpassing your own is unfathomable, but still detectable. That may be lost in translation though: it would explain why Ichigo's friends are able to feel the pressure. Bleach is rather inconsistent when it comes to these things though: I haven't seen someone throw up in a while, for example.