Hi, I have had a LOT more success with the UM2 using 1.75 mm than the 3 mm. I inserted a small 2mm ptfe tube bought on e-bay within the existing 3mm all the way and tighly to the hot head and at the extruder I put a small washer which prevents the smaller tube going back into the extruder. All I can say this printer has never worked better, no more extruder noises, clicks, slips and extruder inconsistencies, a dream come true as I have already got a reprappro Ormerod that works on 1.75mm.
Minor changes to UM2 was changing it to use Custom filament 1.75mm under settings and use 95% flow, yours may be slightly different. Since fitting it I have done many prints flawlessly and have not had a single slip or problem with the extruder. I have also not been required to clean the hot end due to blockage as yet. Whats nice is the 2mm ptfe tube fits really snug into the hot end.
I would recommend this upgrade. Previously I could not push the 3mm filament all the way by hand to the hot end, now I can easily which means the extruder motor uses much less energy to push the filament.
Also dont forget to cool down all the motors within the metal shrouds using a small fan on each side, motors all run luke warm on long prints now whereas previously I couldnt touch them for more than 2 seconds.