Ilyasova can defend Jamison comfortably and do a good job on Hedo Turkoglu. He can also switch onto a wing player and defend them at an adequate level while allowing one of Chicago's wings to take Turkoglu should the Bulls so wish.
The Ilyasova vs Jamison matchup is a wash. An Ilyasova vs Turkoglu matchup may give Phoenix an advantage or may not depending on the remainder of the lineup. It's a small issue, if one at all, for the Suns though.
I think you're overrating Hedo. Again, if he could play the 4, teams would play him there. They don't, and they haven't. He's not a physical enough player to thrive down low.
I think Jamison against Ilyasova is a bigger mismatch than you're letting on, as well (which is the primary reason I switched from backing IP to being on the fence). In four games against Jamison last year, Ilyasova averaged a total of 29 points, being held to 4 points in two of those games. Jamison also had 9+ rebounds in each game, with Ilyasova averaging 4.5 boards per game. Ilyasova picked up 5 fouls in three of those games.
I think there's a real question on whether Ilyasova can guard Jamison consistently, and whether he can keep him off the boards.
(I'd love to see IP address this.)