think in current state 0.5.2 all you rly need is homing + pierce + shield harvest base. later you can expand it ofc. but it makes more sense to just die till you have em in that order. order matter if you plan to prestige. later can replace shield harvest with cooldown reduction stuff with shield leech/multishot/proc chance sockets while still placing shield harvest close enough. dunno. lets see how 0.5.3 or 0.6 will look like.
and yeap once you stop dying most of power comes from weapon upgrades, not xp. you still dump xp into weapon damage/fire rate/shield ofc.
once weapon capped you become too powerful to die if you dont mess your xp/modules badly like taking fluff stuff above useful one