Is CV5 and CV6 programmable for the Mogul, or are they like the GP7 where CV2 is programmable and the others are not?

OK DCC gurus, I need help.

I have a triplet of N scale (of course) GP7's with the factory decoders that I am trying to get speed matched. The first issue I'm running into is their top speed of around 110 mph. No matter what I put into CV5 I cannot get these things to slow down at the top end. CV6 seems to be stuck on 002 and I can't seem to get it to change, either, but at the midrange (speed step 63 of 128) they run right around 67-68. What am I doing wrong? I am assuming it's something I'm doing (or not doing) since all three of them are doing the same thing. They run fine, by the way, I just can't get them to respond to changes in CV5 or 6.

On a side note, I'd love to put some sound into them because I have one of the new SD9's and the sound is awesome, but the only decoder that seems to fill the bill is an obsolete MRC (1955) and I'm less than impressed with the MRC decoder I have in another loco.