it's possible, but there were anomalous power surges before the PED era as well. davey johnson hit 43 HRs in 1973, his next highest was 18.
so it's not really fair to just make that blanket assumption. greene did post some nice doubles totals in his first 2 season, and (historically) young guys who hit doubles often develop some level of power later.
Personally, I think the Cards will get 15ish HRs with an OPS around .680-.700. If they feel that's an upgrade over Izturis - more power to them. With the drop-off in defense (even if it's not substantial) - I'm not so sure. (and if they hit him in front of Pujols, Albert will set a career low for RBIs.)
For $5 million more than Izturis is making this year - I wouldn't have done it. But Greene does have the higher ceiling of the two, no doubt about that.