First of all, lets not start comparing this mod compared to every mod in existence. I don't know about you guys, but while SSP is fun, it just feels like a waste of time for me. So for me, atlest... EE and any other SSP mods are out of the picture, the only mods this should be balanced with is IC2, BC, RP2. Seeing as how the latter two have no variation of an upgraded chest, then this is quite easy.
As for the item id thing, lord why? I just spent an hour converting all my wood chests to gold chests, and then your saying they might not even work properly? Its 2 IDs, honestly... How many spare IDs do you guys need? 90% of the forge based mods resolve most ID space issues, and the rest of the mods are meh? I mean, lets see...
Theres EE, Minefactory, buildcraft addtions, tree forestry, and I don't know what else everyone might be using. Whats really more important? Filling your game with a bunch of stuff you'll probably never use or is probably obsolete due to the ammount of other items you can make which have the same function, or having functional upgraded chests which everyone obviously will use more so then most items.
I like the evolved recipe idea, it will make iron chests more valuable in the start game instead of feeling like a waste of iron after awhile, also, I think 8 wood, 8 iron, 8 gold, 4 diamonds is reasonable for a end game chest.