Most people can not build something so precisely aligned or well made, in particular I'm talking about your 4 rail Z axis.
There are two builds I have found that do a super great job on aluminum, and can also do steel:
https://www.usinages.com/threads/cnc....63531/page-54
http://www.cnczone.com/forums/uncate...uminium-5.html
Linux_fan mentioned his machine's performance improved with the epoxy granite, and both of those machines have filling of some kind, so that is why I mention it. Personally, the machine I'm building will already be too heavy for it, but I started back a while ago and didn't realize all the benefits of it at that time.
This is a Nema23 driven machine?
Personally, I think the 10mm lead is better for all stepper driven axis, with a pneumatic counterbalance for the Z (if it's really heavy) and direct drives on all. Actually, I think 15mm lead is the sweet spot, but that typically isn't available. For servos, I would guess 10mm still but a belt reduction for the drive. I don't think your spindle will have the HP for 100's of pounds of cutting force, what kind is it? So I don't think gearing would be an issue or necessary, but then again I haven't looked at your specific setup in detail.
During the past year I have gotten into the math, including inertia effects.
http://www.cnczone.com/forums/linear...118-cnc-3.html
Graph in post #5 and a sample calculation in post #21
And another example here:
http://www.cnczone.com/forums/diy-cn...45126-cnc.html
Post #27.
With the 5mm lead, you should be able to cut as fast as you would want to in aluminum I think, but the speed for wood and rapids could be faster with 10mm. Alot depends on the specific motors and power supply you have obviously. Really, I'm not trying to bust your chops over anything, just giving an opinion. Your workmanship is fantastic, I really hope that you are able to cut aluminum like a boss with this thing when it is all said and done. Some of the best workmanship I've ever seen posted here.