the 400fps spring was an aftermarket spring that i picked up. the stock spring is about 450fps which should be fine. the pdi hop up chamber comes with a nub but for my setup i found it wasn't enough and i required a bigger one which i cut a piece of o-ring about the same length as the stock nub and added 2-3 small strips of duct tape about the width of the adjustment arms and stuck it to the arms in the little slot which is supposed to contact the nub.
what this does is push the rubber into the barrel with less adjustment required. i'd only recommend this with .36g+ bbs.
as far as slam firing i've never experienced it in my l96 in 3 years of use. my vsr slam fired within 3-4 games so i had to replace the sears in it and i sold it shortly after. the sears on the l96 platform seem to hold up better than the vsr platform OR the setup in the l96 is way less stressful.
from what i've read 6.01 barrels don't really provide better accuracy over a 6.03 just higher fps and i could be wrong about that. its all just practice and seeing what parts work well for the performance/price point you're trying to achieve.