Difficult to say without testing really. But yes, optimization is key once again.
the way I approach optimization was to create a flexible scene doing the following:
nothing on screen, nothing and every 10 sec I had an object. and so on until I see how it starts getting bad. I arrange them in array so that the do not overlap, AND I animate their transform ( it does change the perfs, if you animate or not, so always animate, and nothing set to static.
Then you run that app, again and again, with different materials, mesh optimization. It is tedious, but was the onl approach I could find to gain the experience I need to provide suitable visuals with good perfs.
Only change ONE thing at a time between builds. this is crucial, if you change the mesh adn the mat, you won't know which one had a real effect on the perfs change.
The biggest hurdle we had was actually texture size ( pixel perfect intensive sprites animation, and menus on top of 3d), and scene loading with memory leaks. The mesh and materials caused problems but it was on ipad 1 generation, it went away on ipad2
You need to really bake as much mesh details as you can into a normal map. This require great skills on your 3d soft like max or maya to generate them properly and accuratly, but you'll gain tremendous perfs by just having box with great textures.
For example, check the substance pod tutorial on the asset store and check how much details are backed on the normal map. I know that sama.van on this forum has such skills, contact him, maybe he'll have experience on this and be able to help out possibly.
bye,
Jean