Well, can't say with any certainty at all...
I was on the longest, straightest, flattest "non-drivewayed", unpopulated country stretch (way up in Northern Ontario) that I have EVER seen about a month back. ALL open fields for miles. NO Trees, NO Woods. Just a GIANT stretch of smooth blacktop!! YES - OF COURSE it could NOT be resisted...
I have no idea how long it took to get to 250 - that was a breeze - acceleration changed VERY substantially beyond that though... It seemed to take a very long time to indicate 260...
270 finally arrived though (yes Guys, I know - Indicated) seemed like it took miles! Given that I was inhaling about 3.5 miles per minute, (Yes, the world's truly a blur at that speed) !! It must have been about an 8 mile strip before I shut 'er down... Don't decide to stretch your leg out at that point though!!! The Wind Pressure is incredible!!! BTW, this was with stock shield raised about 75% to "hide" behind...
Given that the "Sweetest Spot" on my '09 is 140-160kph, with 140 falling EXACTLY on 4500, it would THEORETICALLY be possible to hit 280 at the 9K Redline. I think it might need an Edwards Air Force Base length Runway to ever get there though...
That's my 2 cents...
Scott