5th gear on the 6 speed is the same as 4th gear on the 5 speed. 6th gear on the 6speed is slightly shorter than the 5th gear on the 5speed. However, the end diff is slightly taller on the 6speed, making the 5th gear resulting ratio on the 6 speed a bit taller than 4th on the 5speed and resulting 6th gear on the 6speed is a bit taller than 5th gear on the 5speed. This is all with their corresponding final drives. If you combine the 5speed final drive with the 6speed gearbox then you have got the shortest of ratios. If you want lower rpm’s on the high way you’re better off swapping bot 6speed box and final drive. If you do, make sure you get a final drive with lsd if you’re on it anyway 
Source: Roadster.Blog: Roadster NC Transmission & Drivetrain