The biggest chest from iron chests has 4 slots more inventory then the alchemical chest - however its way more expensive to make.
From a balance point of view this is something good, most people don't like EE (and with that the alchemical chest) because its very easy/cheap.
On the flipside, if you're using alchemical chests already then the 4 slot per chest gain will most likely not be a very big incentive to switch over making the mod indeed pretty useless.
As for the weapons being OP - another point of view issue. If you play PvP its totally different compared to when you're not aiming to kill everyone around you and just like a fun gun/weapon to kill creepers or other non-player nuisances.
