I have had a disco t/box fitted to my 300tdi for 6 months now.
Haven't noticed any loss of acceleration, but it doesn't cruise as happily on the motorway now and even the slightest of hills needs me to change down to 4th, even when I'm doing 70mph. If you are on the flat or going downhill it is fine, and seems to rev lots less, but I am not sure it is worth the effort as 4th on a motorway is just wrong!