Honestly I dont think that you could climb much in 6th, maybe if you had lower gears out back then maybe it would offset the extreme overdrive ratio that the 6th gear has.
If there wasnt any wind resistance on the car then you could in theory keep going until the tach hit the red, but because of that wind resistance it puts a really heavy load on the engine, thats why you cant keep pulling in 6th. The power output with that gear reduction isnt physically possible.

 

Out riding with some of my buddies with vettes and vipers and we were at about 180 but out in the desert there is lots of empty stretches here and you have to keep it in 5th once you shift to 6th it doesn't climb hardly at all but 5th still had a bit of pull. 4th was pretty peaked at that speed. This is just going off of what they were saying. And let me tell you these cars might look aerodynamic but they are really not. It feels like total **** at those speeds!!!

 

My best was a little bit over 5200 rpms in 4th with 3,23 axle and 245/50/16 Tires. The calculater says 175 mph. Not bad for stock

 

I've been to 162 when I hit the old speed limiter. It's been raised since then to 200. I had no problem climbing through the R's in 6th gear but I have 4:11s and a few other things helping me.