If he was the PM, then I'd be worried, but then again, he is the PM, sort of.
That's the issue. Gilani owes his support from the PPP to nothing but his blessing from Zardari, who the party is loyal to. That makes Zardari essentially control the PPP.
In a Zardari presidency with Gilani as PM.... we will effectively be seeing the merger of the Presidency with the prime minister's office.
Indeed, I don't really see Zardari relinquishing any presidential powers, meaning 582b would remain in effect. The more power for him, the better, he will feel.