There's no real significance to the 2 mm difference in diameter. It's the kind of incremental change that an OEM makes when trying to find the most "balanced" way to add power. Honda does't have to start with a GSR throttle and replace it with an ITR one, they just equip the ITR with a different throttle from the start.

If you want to upgrade the throttle on a GSR you're going to have to buy an entire replacement unit so you might as well get something with a wider throat, say around 70 mm. Erick's Racing makes a really nice 70 mm piece.

But really, the throttle is not much of a restriction and I can think of a whole lot of other things I'd bother to replace first.