The US has never had a class system.
Sure it has. Maybe it hasn't been as explicitly defined, but doesn't mean it didn't/doesn't exist. Heck, we even use the word "class" to describe income brackets.
"Human, your request is limited by your physiology governed by the laws of physics. Shed your flesh and embrace the vessel of new construction based on electrons and photons."
While there are certainly biological limits, I highly doubt what we have now is anywhere near the maximum of what is possible if we were designed intelligently rather than by random mutations. It might require wholesale rewrite of our DNA and associated cellular mechanics, but I'm sure that there is plenty of room for improvement. A more likely response would be the AI asking questions about trade-offs that you are willing to accept in order to achieve your goals. For example: Increasing your intelligence beyond X% will require expanding your cranial cavity by Y%, and increasing your daily caloric consumption by Z%. Is this acceptable?