The way I see it, the uni can point higher because the main can be sheeted to centreline of the boat, whereas a sloop has a jib which can only be sheeted off centreline, otherwise it would stall the main.
The fact that the jibs angle of attack is different to that of the main limits your ability to point. If you move your jib fairleads inboard (without closing the slot) you should be able to point higher on a sloop.
Then again, I`m no Hydro-aero-cosmo-nologist, so don`t quote my theory in your book.