First clean your barrel/bucking
then use a heavier weight bb, say 0.25 upto a max of 0.28-0.3's (they get a lot more expensive, and the benifits are minimal on 350fps)
then an upgrade your hop rubber (I personally like g&g green or a tm rubber)
then look into hop units/barrels etc etc
why this order ... simplicity and cheapness
See I personally wouldn't spend anymore upgrading the delta, I'd save my money and buy a higher end gun.
To get the most out of your gun, sp00ns advice is best.
You don't want to be spending fortunes on a cheap gun.