http://www.mintylamb.co.uk/gearspeed/ will show you the ratios, my 1850HL box: 3rd OD had a 0.01 of a ratio higher than 4th or so the manual dictated, so there was no noticeable difference at all!
The sprint gearbox however, 3rd OD is noticeably different to 4th
30mph in 3rd OD is around 1700rpm and 4th is around 1550rpm to give you an idea, however I personally just go 1st, 2nd, 3rd, 4th, 4OD as 30 in 4th seems the most efficient for the engine, on track changing from 3rd OD to 4th is a much more useful option as it is much more noticeable at higher speeds, it also allows for a faster acceleration towards the standard top speed and when you change into 4th you won't have drifted too far out of the peak power band to continue accelerating, but this is at engine speeds of around 6000rpm which I'm sure you don't drag the dear 1850 engine to and, being an 1850 it will be no different at any engine speed
Another bonus of a sprint box is that it is impeccably easy to wire in OD for 2nd which is fantastic for slow moving traffic of around 15-20mph!
Just be careful if you do switch out of OD whilst changing gear, as if it is mistimed, you'll feel it!