Hey,
yes the ISTC edition speedo does fit, I used to use the Nosram version in my pred, x10 and x11. If you look at the pic linked below, you'll see that to get the shell on comfortably over the fan, you need to undo the screws holding it on the heatsink, move the fan back and then refit the 2 front screws... 2 screws will hold it on fine, you save a little weight too I guess.
http://www.oople.com/rc/photos/buryn.../NO5E9047.html
Yardy