From a theoretical side, the smaller SP1 T-B's should allow higher velocity air flow than the SP2's.
This is ideal for low to middle throttle settings.
On carburated engines, too large a throttle bore meant flow stagnation at lower revs and throttle openings.
So on an engine with huge throttle bores, if you wacked the throttle opened at low revs, it would spit back when the flow stagnated.
Also higher flow velocity means greater opportunity for ram effect at lower revs.
This was one of Suzuki's secrets to mid-range power on it's GSXR's for years.
Honda increased the SP2 race bike T-B's to 62mm from 54mm for greater top-end HP.