I found that the additional mass of the Bowden setup (hot end, adapter, extended carriage) really made no difference to the effective inertial mass (it's more, but not enough more to make a difference.)
Dual bowdens is definitely the lightest/fastest approach. My concern, and it's turning out to be valid, is that most of the time I'm not printing dual extruder prints. I want it when it need it, but for a lot of what I print I want to get the best cleanest single material print possible. For those, I want the stock wade's, not a Bowden print. The tuning to get the Bowden quality on par with the stock extruder is murder.
I haven't tried a tube smaller than 4mm for 3mm filament. I do run a 2mm tube for my 1.75mm filament. I'm a believer that the excess tube diameter is the major contributor to ooze, but even PTFE isn't friction free and when the carriage is at the position where the tube is at max bend, too tight a tube is going to max bind at that point and I don't know when that's too much.
(optional Dual Extrusion) Printrbot Plus V1 [3/2013]
bed stabilizers, bed levelers, Y & X belt tensioners,
50mm fan mount, LED ring light, Z-axis screw Isolators,
Z-shaft stabilizers,optional Bowden conversion, GT2 belts/pulleys,
M8x1.25 Z Screws, Flex. Z-couplers,
RAMPS 1.4 w/ LCD,
Quick Change Extruders:
Option 1- Dual hybrid geared Wade's/Bowden w/ 3mm & 1.75mm Ubis,
Option 2 - Dual direct geared Wade's w/ 3mm & 1.75mm Ubis
Option 3 - Single geared Wade's w/ 3mm or 1.75mm Ubis
Option 4 - Single belt drive Wade's w/ E3D
MIC-6 metal bed
Thingiverse Profile: http://www.thingiverse.com/tdeagan/designs