Yeah that's what I'm hoping for too
I have the original iPhone 4, and it's also quite full. Slowdown is particularly noticeable on the results screen where the critical bars are populating, or when I've been playing it for more than two or three songs in a row. If I play too much I'll get terrible lag in the middle of a section and the game just counts them all as "missed" even if my inputs are correct.
I've still managed to get a S rank on some of the ultimates and SS on most of the expert scores though, so I still enjoy it.
The best way I've found to play it is to close all of the apps on the phone, put your phone on airplane mode (aside from the fact that getting any form of notification completely ruins your score, airplane mode significantly speeds up the processing power of the phone in games) and make sure you're starting the game fresh. Meaning it's been closed and you're re-opening it. That way it runs flawlessly for about 3 songs before it starts lagging again.
Having the original 4 also means I don't get full backgrounds, I get mini versions on a weird looking green screen
.
But yeah I'd be really really excited to pick up a version that has all the 3DS stuff (FMV events etc) as well as all the extras on iOS. Especially if it was for the Vita.
EDIT: The only reason I still have the original 4 is that I used to work for O2 and I got a staff contract, which is unlimited everything (including internet) for £15 a month. I'm so not ready to give that up, the second I upgrade my staff contract disappears since I no longer work there