This is purely my personal experience as these our are first brushless systems. Both my son & I run the Nosram Matrix Evolution 2007 version speedo's, mine with an LRP X11 6.5 & his with the Nosram Storm 6.5. Mines fitted in a HB Cyclone D4, has the factory heatsink and only the roof vent cut out & his is in a Ttech Predator XRS, no heatsink fitted and both side vents cut out (this is much tighter for space than the D4).
So far we've had no overheating issues although I shall be doing another shell with vents cut in the front for the summer. We're both running overall gearing around 12.5. I shall also be ordering the fans from the TC version just in case.
I appreciate what Modelimages is saying and we are only club racers doing the odd away meeting but so far we've been very happy with what we've bought.
We bought ours from our LHS as we know that if we do have any problems they'll support us if the need arises.