Upgrade SD2 or build...

Ok from experience has anyone maxed out their SD2 with some real hardware upgrades to improve qyuality and speed?

like bearing carriages, bigger steppers, linear slides and did you see any major improvement in speed or quality

If you were to build a new unit and now with the SD2 in hand it is cheaper to make reraps.

Which ones com close to their published speeds, have improved print quality .1m, and are more reliable and less prone to the finicky adjustments?

Rostock - 800mm/sec?
MendelMax- 200mm/sec
Prusa

I know I am printing at 35mm/s maybe 60 on some perimeter only models.  Quality seems pretty good to me but I can see this not that reliable of a unit lots of calibration. Other units look more robust. And by printing the parts, getting $100 of some aluminum and pulleys and $200 in electronics I could have a second unit.

I was thinking very simple, very fast and  more reliable unit that you build at home for $500. If I had to go up to $1000-1500 I guess I could get off the shelf prebuilts mentioned here.