Originally Posted by Drifty

I dont see any negatives with a 6 speed gearbox.The only disadvantage will be the added weight in a 6 speed gearbox .A vehicle with a 6th gear will be able to cruise at higher speeds at lower rpms without stressing the engine as compared to 5 speed gbox.It can be related to something like torque for example, a bike with good torque often doesn't require more gears because you'll lose time through changing, or gain no benefit in other words.
Or a bike with comparatively little torque can benefit from an increased ratio range.
Sometimes 6th gear is used as an overdrive gear(as seen in the Suzuki GS150R)
so that the previous gears can be narrower to allow increased acceleration (and thus performance) in the lower gears, but still be
able to obtain good speeds whilst
achieving more economy figures.
BTW the gear ratios of RTR180 are ultra short and we always get the feel of having a 6th cog! ;-)