Bowden, Bed and New Hotend Benefits, real world?

With a good hotend and a few tweaks here and there I see no need for making any change to the existing extruder, at least as it pertains to the SD3. I swapped to an E3D and did proper calibration of the filament feed as well as adjusting the pot on the Printrboard for the smoothest possible flow. After changing the pulleys and belts to GT2 16 tooth I have to say I have zero issues with the way the prints look. The consistency of not only print quality but dimensions of the printed objects is, IMO, vastly improved with proper adjustment and a high quality hotend like the E3D.

IMO a Bowden is NOT an improvement, they tend to ooze a lot without using carefully calibrated retraction to get the best results, a PITA matter of trial and error really. I built a small Delta that requires use of a Bowden setup due to the nature of the Delta design, even with an E3D hotend on it I still can't get the same high level of print quality from the Delta that I can on my SD3 although the geared drive on the Bowden does completely eliminate any visible moire in the print so I see it as a tradeoff of overall print quality vs. how it looks. I'm sure some of it is the SD linear motion design vs. the delta design but the Bowden system doesn't seem to be the better extruder for print quality, only better in terms of not having to move the weight of a stepper motor around with the printhead.

That said, my current project involves an SD3, a pair of J-head hotends, and a pair or Bowden extruders. Mainly an experiment to see if it's practical on the SD box and how well it works,...  or not...

I don't consider the J-Head a huge improvement, at least not the Chinese clone ones which is what it turns out I have. I'm setting them up on a dual extruder carriage for the SD3 which is proving to be a much bigger project than I originally thought it would be...

Unless you're prepared to do a full upgrade to bearings on ALL shafts, and that includes the X and Y axis idlers along with GT2 belts and pulleys, I don't know how much quicker/faster you'll be able to run and still have the same or better print quality. Solid print quality and repeatability is based on the premise that the head is going to extrude filament at a consistent flow rate and temperature and the carriage assemblies are going to be able to position that head with extreme precision every time, time after time. The only way I know of to make that happen is to eliminate the slop on all axis and the quickest route to accomplish that is to make anything that turns or slides do so in good quality bearings on straight precision ground shafts and that all assemblies are squared up and properly aligned to their respective axis.

I've spent a considerable amount of time doing exactly that on my SD3 and the end result is that it can print with some incredibly crazy acceleration numbers and still have good print quality. Most people think it's the MM/second number that makes a printer fast and it does to a point, but a better measurement of print quality and printer speed is just how quickly you can accelerate/decelerate the head to/from those velocities while maintaining precise head positioning and that takes a lot more than nylon sliders, bronze bushings, and mill cut rods.

After doing all that, a good hotend is just the icing on the cake and by itself isn't going to make a huge difference in print quality at higher speeds if it's flopping all around on the X and Y axis.

They are in the code in the configuration.h file but the easiest way to see what you have and change them is open the Firmware EEprom configuration in Repetier from the Config tab, they are one of the parameters that can be changed without recompiling the code, or at least it's possible with the Printerboard on the SD3, don't know if the capability exists on the older SD2s as I've never owned one.

I've actually run mine with the max acceleration set to the Marlin default of 9000! If you want to see a hotend move absurdly fast even at 60mm/second speeds, bump the SD default setting from 1000 to 9000, but be prepared to catch flying parts unless you've done some upgrades