I've been really think about doing the Openbuilds V rail upgrades, but first I am going to get some lead screws and raise the Z axis up. I may also consider an extruder upgrade in the future, but one step at a time
The Y rail upgrade has been GREAT. I can see a difference in my prints Y vs. X axis now. There's noticeably less ringing in the Y axis than the X...and it's SO MUCH quieter now. I need to revisit the modified accelration/jerk settings I was using when first trying Marlin 1.1.x that had almost eliminated any ringing on both axes. I also need to adjust the belt tension on Y as it's stretched a bit after a week or two of use and is a little sloppy now, but that means taking the bed off and that would mean rebuilding my UBL mesh and I haven't felt like dealing with that yet. Once the aluminum bed arrives it will be worthwhile to make the change I figure. Just not sure if I'll still use glass with the aluminum or not...I can't afford to put PEI or Kapton on it right now and don't want to print directly on the AL...so I'll probably keep the glass for now. But it will mean giving up 3mm of vertical height (which I basically never use.)
If you do the lead screws get the anti-blacklash nuts...they weren't available when I did mine and I wish they were because I do have a bit of backlash that affects my Z on some prints. I just ordered a set which I plan on installing when the new X parts are finished.
The extruder upgrade...honestly it was probably the worst upgrade I've done. I got better quality prints with the original MK8 style extruder and the $10 aluminum extruder body with adjustable tension I got off ebay. The reason I "upgraded" was to get the higher temp range of the v6 hotend so I could do PETG better and Nylon (maybe someday even try polycarbonate). The new extruder is also better for flex since it has more constraint. There are ways to mount the V6 to the original plastic extruder...but the original plastic didn't work well for me since the pivot was wallowing out and it didn't have adjustable tension - and the design of the aluminum version had no way to mount a V6 hot end. I really wish I could find an aluminum mk8 with adjustable tension - but without the big aluminum block on the bottom that prevents installing a v6.
I'm looking at moving to a geared extruder to lessen the forces on my extruder stepper since it runs hotter than any of my other steppers. Just been a low priority change since what I have now works.
I may just redesign what I have now to mount better on the V rail and to shorten the distance between the drive gear and the hotend which is my biggest complaint about it and what I suspect is the cause of the slight drop in quality I've experienced since it's less direct of a direct drive and more like a solid bowden that's only 1.5" long.