Looks like your going for a standard footslogger army. Some of your stuff doesnt really suit it however.
Kff mek - always useful, especially for obscuring your walkers. Drop the armour though, itwont help during the shooting phase and he should be out of assault range as much as possible anyway.
Maybe give the shoota boys big shootas? Won't hurt if you have points laying around.
Good number of lootas for a 1.5k game.
Deffkoptas - I'm not too sure about these. They get to the enemy far quicker than the rest of your footsloggers, so there just going to be mashed up before they earn there points. Bear in mind, your likely only going to get one round of shooting from them (with tl rokkits, this amounts to 3 hits on average.) I think you'd be better off replacing with kills kana - they can take advantage of the kff obscuring rule, as well as provide a 4+ to your footslogging boyz! It's also worth remembering that you do have 20 lootas - there going to put a lot more hurt on light veichles than the koptas will, and are almost guaranteed to survive multiple turns, whilst also allowing you to hammer on side armour with clever positioning.
Killa kans - big fat juicy poppable target. Drop them or get more, at 1.5k points these will fall too quickly to be of use unless you have too many for the enemy to handle.
Big gunz - zzapp guns at great fun, but rather unreliable at taking out veihicles - especially given you already have a good amount of anti-tank already. If your using them for fun, go ahead, but the template big gunz (forgot the name) is cheaper and more likely to whittle down the enemies infantry.
Seems a solid list overall, looks like great fun to play, but if you want it to be competitive you'll need to specialise a little more.