Yeah, not really. Heimer's spare parts do have a longer CD than seeds, but it's nowhere near double.
Heimer: Recharge Time: 24 / 23 / 22 / 21 / 20
Max CDR: 12
Zyra: Recharge Time: 16.7 / 15.4 / 14.1 / 12.9 / 11.7
Max CDR: 7
You're right that Zyra's plants are "disposable" because they're short duration, weak, short range, and (still) stupid as fuck. They're also very large, very obvious, and very loud. By comparison, Heimer's turrets are only remotely dangerous-looking when they shoot the blue beam for 45% of his AP.
Spawn timer aside, Zyra's plants cost way more mana than it does for Heimer. It costs Zyra 75-95 mana to spawn a thornspitter, and it only costs Heimer a whopping 20. He can drop all 3 turrets from his reserve at rank 5 for less than it costs for Zyra to spawn a single plant at level 2.
My point is that there's no special, bizarre distinction here that explains why Zyra's plants are so much worse. Her burst isn't higher. Her range isn't higher. Her sustain damage isn't higher. Her scaling isn't higher. Her base damages aren't higher. She's not more durable. She isn't less mana reliant. She's not faster.
There is literally nothing Zyra can do that Heimer can't do better outside of lucky or very well-planned wombos with a Yasuo. Even then, a Yasuo would rather have a Malphite for a wombo since Malph's ulti is longer range and is actually guaranteed to both hit and knock up. As much as I love Zyra, there's no purpose for her to exist while Heimer's around.