I meant to reply to this yesterday.
He's saying that it's easier to get a +3 with a 6/9 than it is with a 5/8. However, in AS, that +3 is always there, regardless of movement type, making 6/9 less relatively useful when it comes to defensive mods than it would initially appear compared to 5/8.
This is what he was concerned about. However. A mech with a +3 movement modifier doesn't get to do the obscene amounts of damage that the Mad Cat III does. Its intentions were to be a miniature madcat and it does the same damage twenty tons lighter. (The Standard does. The X is a monster on the AlphaStrike table)
On strategic level gameplay, you are only going to be able to turn twice and have to run to get a +3 movement modifier... but as has been mentioned...
Where this 'Mech really wants to be is in your cavalry formations grouped with 5/8 designs like the Timber Wolf to serve as an inner screen just ahead (meaning within weapons range) of your core assault element to blunt attacks while on the march and redeploy as needed in combat.
This is where this medium belongs. In your heavy lances for fire support. Its got enough guns where if they run forward to get you its a bad idea and for the most part can out run anything bigger than itself that wants to get at it.
Its got two brawling variants as well. Though one is someones personal ride. Its terrifying.
There are only four fifty five ton mechs that move faster than 6/9. Its only five tons away from being a heavy mech. So in this case Medium mech doesn't = scout. Generally medium mechs try to add some kind of utility. Since your best scouts are going to be in your light category. The only difference being the low end of Medium tends to provide escorts for lighter mech formations.
But beyond that you get mechs like the trebuchet or whitworth which are just missile boats. Hunchbacks which are close assault mechs.
Centurions, Gauntlets, Quasimodo's and Wendigo's haven't arrived to find you...