Why would I want them on an Ostscout with a single ML when I can have them on a Venom with 3 MPL?
The Ostscout already jumps 8 anyway, and you can't jump more then your walking speed with normal jumpjets. Improved ones are heavy. Looking at currently (3077) available Ostscouts there is the 10CS which has 10 improved JJ, 2 med lasers and a C3i computer and comes with double heatsinks. But I still prefer my Venom.
I have 6 active lances (one of them an aerospace lance) and I only use a single assault mech. Mostly because I needed some LBX weapon against those pescy hovercraft the Word of Blake seems to love to field and it was the best thing I had available.
Most assaults still move 3/5 or 4/6 so I am not sure what you mean by making it easier for them to be mobile. XL Engines? But you pay for this with lowered survivability and less space in the chassis. And small mechs can have them as well. Those hovercrafts I mentioned were on a swampy map where they could zip around and almost always get a +4 to hit from movement while I was happy when I got +2, mostly it was +1 or +0. I only won that when my air support arrived on turn 5 (still lost a hover of my own, had to eject a pilot from a Lineholder that had lost a side torso and had all of 7 armor left, most of that on the head, and had to finish without a Rifleman when its light Gauss went boom and the pilot auto ejected. Only had to replace the side torso though.)
Most assaults I have in storage are Awesomes for some reason, the first three I salvaged were all Awesomes.
I prefer medium and heavy mechs, I field 2 light ones at the moment, and one is in a training lance because it was the only thing that fit without going over the weight limit.