Thanks for the update.

I've been tinkering with mine too, it's running well, but not as well as yours. Can't believe you printed the dragonfly at 100mm per second and it's come out great!

I set both my x and y axis at 80 steps per mm and have accurate results. Since the belt pulleys on each axis are the same, in theory the steps per mm should also be the same. Have you checked your pulleys to see if there's a bit of crud on them or if one is slightly deformed?

What thermistor did you specify in the marlin config? I'm not convinced I'm using the correct one.

How did you modify the extruder for flexible filament? Did you just put some ptfe in that small gap between the extruder and the hot end mount?

I've got more tinkering to do over the next few months and I'll post an update of what I've added. So far I've added:
- reprap discount screen + ramps 1.6 board (a ball ache to change and re-configure, but worth it to get rid of that horrid lcd)
- Replaced threaded rods with T8 leadscrews + flexi couplers (seems to run smoother, but hasn't really made a noticeable improvement)
- PTFE tubing
- A cheap TE917 build plate (the PLA adheres brilliantly but it's a bogger to remove the print)
- Added an extruder fan (this probably made the biggest difference to the print quality)

I did try to upgrade the extruder to enable me to use 2.85mm filament since I have 2 rolls of the stuff. I eventually realised it's not worth the hassle.

Still to do:
- Fix the whole thing to a slab of 18mm MDF (to give the frame more rigidity and to make it easy to move the printer without having to calibrate)
- Add power switch
- Add cooling fan duct
- Add bed levelling sensor
- Build solid hood (for covering the printer while stored and to ventilate abs printing)