Yeah, I guess that's what they meant by having a caster dps carry. Nothing is too good early, but as you progress through the game, the overload flat mana cost will carry him to becoming a spell machinegun. Same for the AP ratio, which is **** early game, but as you get the mana regen to sustain casting every 3.5 seconds, that even it out. What to pick early? I guess overload remains the main choice, as this is the first thing you want to max out. As it gets more damage output, it keep the same low cooldown and mana cost.
EDIT: Ew, that and I just remembered that his passive is: each time you cast, the cooldown of other spells is reduced by 1. So really, as you will spam overload, the REAL cooldown of the other spells is really less than their displayed value. I mean, you can cast overload 3 times before the bounce is ready, meaning its cooldown is actually 11 seconds rather than 14 seconds. Then imagine you get the 40% cooldown cap (say blue buff + item), your 3.5 seconds is really 2.1 seconds, and everytime you cast it, you remove one extra second from your 8.4 seconds cooldown rune prison and spell flux, which in turn whenever they are cast almost refresh the overload cooldown.
This also means that the 50 seconds cooldown on his ultimate is much shorter, and will be much more spamable. Again, assuming 40% cooldown, this means that if you cast Overload and other spells about 10 times in 20 seconds, the ultimate is ready after that 20 seconds (50 secs * 40% = 30 secs, -10 seconds for spaming other spells).
Obviously this is all theorycraft, and you need some insane mana regen to sustain this, but his outburst is there, and it works in theory ^_^