Johnny, the Si has a speed limiter @ 133mph. Thats basically right at the end of 5th gear, rendering 6th completely useless.
Now that the engine is modified and the speed limiter removed, i've taken it to the mid 140s plenty of times. I haven't tried to top it out with the limiter disabled. The OEM tires aren't rated for those speeds and i wasn't about to push them.
Now that i have Brand new Toyo T1R's that are rated to 169mph i can explore the upper end of 6th gear on the open road.
Mechanically and theoretically, it has enough gearing to reach 180mph.
Just an FYI, i topped out my Ap2 S2000 @ 157mph and that was redlining 6th gear @ 8,000rpm.