What do the rest of you Ork players do when the opponent castles? I know people say charge forward and envelope them, but it feels like this just gets the tar kicked out of me. I wonder if this would be less of the case with bigger units taken compared to my previously tried numerous small ones?

Well concerning castles I use soopa-gunz. MW barrage attacks are great for killing infantry in cover (or outside of it ). I usually play with a sups stompa and at least one other formation with two oddboyz (warband or big gunz) at 3k. The stompa mob might not get to do a lot the first turn, but if they double up the middle then they usually will have one or a few targets to shoot at from there.

I have found the key to get the soopa guns to work then is to try to sustain 3+ with a reroll is actually a roll that you will make most of the time. Even if you fail then you shoot and hit without the minus one penalty you would have had if you would have doubled.

I'm going to add one thing, just because you haven't explicitly mentioned it. If you're going to spam smaller formations don't forget you can prep targets before an assault, double up with some scorchas, fire, then assault with a second formation and get the supporting fire on top for a second round of shooting. It can be really effective.

So if you're packing 3k points, how do you compare 500 points for a stompa mob vs 650 points for a Gargant with the Warlord inside? My biggest corncern with the Stompa Mob is that Titan killers can take out the 3 Stompas and break the Supa Stompa without having to even touch the Supa Stompa itself!

I also don't seem to use Warbikes very well. I tend to take them with one buggy for dropping Blast Markers, but they tend to not do much. I should probably try to keep them on the extreme flank perhaps? They really only seem to do well against softer support targets.

Well concerning castles I use soopa-gunz. MW barrage attacks are great for killing infantry in cover (or outside of it ). I usually play with a sups stompa and at least one other formation with two oddboyz (warband or big gunz) at 3k. The stompa mob might not get to do a lot the first turn, but if they double up the middle then they usually will have one or a few targets to shoot at from there.

I have found the key to get the soopa guns to work then is to try to sustain 3+ with a reroll is actually a roll that you will make most of the time. Even if you fail then you shoot and hit without the minus one penalty you would have had if you would have doubled.

Yeah, the two superguns with a sustain order is a great order for making big dents in big formations.
I thought though that if you did fail your order roll you get a -1 to hit. (same as a double move)

To Hit Modifier Table
Target is in cover
-1
Attacker is carrying out a double or marshal action
-1
Attacker is carrying out a sustained fire action
+1

Hold: This action is the only one allowed to a formation that fails an Action test (see the rules for action tests next). The formation may make one move or shoot or regroup. You must choose to move if any units are out of formation.

I can see how the regular stompas might seem like a liability, but I find them to be a bonus instead they can soak up TK (DX) hits if put in front of the stompa (since the extra damage from a hit doesn't spill over and is then wasted). This is something that a gargant cannot do (although it does get more shields).