gkn overdrive

I am thinking of fitting a GKN /Laycock overdrive to my 2005 defender 90 td5. Have any of you good folks used one? If yes how much difference does it make when cruising on m/way. Is it something I could fit on my drive or do I need a ramp?

I haven't tried a GKN, but I fitted a Roverdrive/Roamerdrive at the end of 2015 and wrote about it here: https://www.landyzone.co.uk/land-rover/building-the-gearbox-of-my-dreams.293835/ The job's a bit more complex in my case because I changed the diff in the transfer case while I was at it. I did mine at the side of the road over Christmas so I'm sure you can do it.