So a couple of things.
Where are the nobz with power klaws and boss poles? Without them your boyz won't fare very well against a lot of enemies.
Secondly, thought the mega armour warboss is great and can deflect a lot of shots, you won't be able to run with that squad, so, though I love the armour, I would downgrade to Eavy armour so you can still get that WAAAGH off.
I am not sure big mek
KFF is worth it since you can get almost 2 pain boyz for that and give the two slugga squads feel no pain. Because the
KFF only works on models within 6" rather than squads within 6" which means the feel no pain may be better. Having said that, your list might just die to vindicator a unless you can pop them quick. I would possibly drop in some tank bustas in a battlewagon instead of the deff dread and and zap guns, and possibly the looters as well because I firmly believe that the tankbustas will pop more things.
The problem is that you need to pop a lot of vehicles in order for your dudes to charge the squishy goodies inside. Hence the tankbustas
Hope this helps
A few of my points are invalid of course if you are using the old ork codex, but I'm assuming your not?