A while back I saw a youtube video of a japanese guy who was printing at like 300+ mm per second on a solidoodle. As far as I could tell all he did was use aluminum parts instead of abs that came with the printer. So I got to thinking maybe carbon fiber abs parts at 50% infil would be lighter and just as strong and help your print head move faster too? Has anyone ever tried it or have an opinion?

1) I have seen that and will at some point make the aluminum carriages. Just a matter of finding the time and motivation. As I work full time have a  house and family and I have been putting in some volunteer hours at a local maker space.
2) If in fact all the things in the video are true  300 mm/sec on a SD is possible.
3) HAs Anyone here printed CF carriages ? We will have to wait for that someone to respond!
4) This is an open forum Opinions are abundant.  and often varied and sometimes opposing.
5) The lawsey carriage Upgrade is a popular and recommended upgrade for the SD printer.
  a) IMHO no reason not  to print the carriages in a strong light material .
b) Clororfab XT carbon would be my first choice. ( i have not tried it but excellent rep)
  c) colorfab products available from http://printedsolid.com/pages/catalog
  Note :make sure you extruder temp is high enough for any material you use or purchase. 
6)Carl is correct the extruder and hot end need to be able to put out material  to keep up.
7) There are many factors in a fast accurate machine. The mass of the carriages and print head are just one.
8) With a small envelope it is acceleration you need to worry about. The less mass you are moving  the easier (less force) to accelerate.
9) increased stiffness  of the frame and will help dampen vibrations from high acceleration.
10 ) The ED3 volcano is designed increased deposition rates.
11) a .6  or .8 mm nozzle will significantly increase plastic flow and deposition rates.
12) if you are going to upgrade  and looking for increased performance consider upgrading to an ED3v6 hot end as well.

Tin

Doesn't stability and weight also become an issue if you go really fast?

It does, but it is not the main issue. You could have a machine made of spider silk and carbon fiber, but it still will only go as fast as you can melt and lay filament.

A while back I saw a youtube video of a japanese guy who was printing at like 300+ mm per second on a solidoodle. As far as I could tell all he did was use aluminum parts instead of abs that came with the printer. So I got to thinking maybe carbon fiber abs parts at 50% infil would be lighter and just as strong and help your print head move faster too? Has anyone ever tried it or have an opinion?

You can make your head move any speed you want. The issue is can your extruder and heater keep up and feed the filament at that speed?