Given vmax is only a few mph more, it makes you wonder what sixth is for

I think there is every chance that when you change up, it will slow down ....
There are only three places at the ring where you might get sixth (I won't see it anywhere!).
Along the straight from Flugplatz to Schwedenkreuz - in the S1 I see nearly 130 there - in the Exige I reckon there may be a bit more in it, especially if you brake later and take more speed into Schwedenkreuz (100mph on the apex is normal for my modest pace). I would be surprised if you manage to squeeze in a change up to sixth, but I guess if you carry a lot more speed through the flugplatz that could happen, just. On my one trip there with the Exige (and with the 48s) I was exiting flugplatz just on cam in 5th, so you would need to be well into the cam to make it.
The start/finish straight is ~two miles long, though of course you cannot use it all on tourist days. Even so, if you had a shot at it, you might just make sixth, but crashing through the cones there is likely to lead to being banned
The other place is the straight after Pflantzgarten III - I don't go full bore until after the undulations and can make 120 at the end with plenty of margin for braking/turnin to Schwalbenschwanz (sp?). If you were on it from the crest of PGIII the whole length of that straight and didn't lose control/go airborne in the slalom/wave section, you could be seeing 140, but sixth? Doubt it.
A place where I don't think you'll see sixth is the Fuchsrohre - it is very fast and even 1.6 saloons see 120 down there - but I think it's just too short to top out fifth AND make the compression/corner at the bottom! But with big balls, who knows?
---
I have seen terrible crashes at each of these places, BTW, and I'll bet *none* of them were doing 147mph ...
Robin