Should I shift my project to Unity 4?

How's that for an open-ended question? :)

I've been working on a project in Unity Pro 3.5 since about August of 2012. Things were initially going fairly well building and testing in the Web Player, but recently I've spent more time optimizing for the iPad (which is one of the target platforms). Things are mostly in good working order with the exception of the original iPad. It's questionable as to whether I will support it because it's slow and often crashes because the device has limited memory.

I know that Unity 4 has some improvements in the mobile area. But I have been hesitant to upgrade because the project has been long underway and progress has been good...I don't really want to "shake up" my development environment and introduce unknowns and risk.

But if Unity 4 will improve the game's performance and memory utilization on iOS devices then it might solve some problems, give me more exposure to gamers that still have an iPad 1, and be worth the risk.

Has anyone out there undertaken an iOS project upgrade from 3.5 to 4? What were some of the challenges you experienced? Did you notice any improvements in resource utilization and/or performance?

Thanks!