Was a fun game. I was thinking about your list again, and what I said about making all of your infantry hit one place at the same time. I started second guessing myself and thinking that alone they're just don't pack enough punch by themselves. However, if you paired them with the flame tanks of death, I think that would be the most effective. Think of this: your tanks (which, in reality, your tanks>any tanks) focus on popping transports. Your flame tanks roll up and toast the meat that falls out. Your mech infantry tag along and get out and rapid fire or assault (if you're near an objective) or they keep trucking to the next target. I think 'divide and conquer' is your biggest ally with that list.
I really don't think it needs to change much. And let's be honest, that melta gunner had no right to be alive that long, nor should a 20 point model be popping 2 tanks. I think you just have to watch out for deep strikers/outflankers.