Never with a 4 speed MORGAN, but with my 2005 PLUS 4 with the 80km/h and former 90 km/h speed limitation in France I found that 5th gear at 2200 rpm is too low and I generaly use the 4th (2500/2900 rpm). Sport cars must have short and close ratios. 3000 rpm is half of the maximum rev and doesn't stress the motor. If you always use low rev your motor will stay lazy with poor accelerations and cannot get its full nominal power.
That sounds frustrating.
However, as mentioned before, the 5spd box is exactly the same ratios at the 4spd (1 to 4), so will have no change to acceleration, but it gives you an extra gear for cruising. Means you can travel at motorway speeds (110km/h) without revving the tits off the engine.
