iPlayer is very powerful in terms of raw CPU speed, so much that it doesn't NEED the DSi's extra specs, it outperforms both DS and DSi. It has a 400mhz internal CPU, which works on top of the DS' 67mhz main processor. It's also said to have a secondary ARM CPU with around 200mhz extra according to specs. And it has like 32MB of extra memory.
And yeah, it works on both DS and DSi. And no benefit comes from playing it on a DSi over a DS, it is locked at DS speed. You'll see no performance difference. You could say that the iPlayer was created in response to the lack of DSi hacking, to supplement the inability of being able to tap DSi's CPU and memory. That's really all the DSi is anyways- an overclocked DS with more memory. iPlayer steps that up a notch.
And there is some speculation that the guy who made the iPlayer GBA emulator is in fact working for Supercard team. He managed to get an iPlayer development kit from "someone" according to him. No one has the devkit but him either. And now there's news that the DSTWO will be getting a GBA emulator of its own (it shares the same specs as iPlayer and iPlayer was made by Supercard anyways). If he wasn't a direct employee of Supercard, i'd guess he is now and is now developing DSTWO's GBA emulator (which i speculate to be a simply updated and improved version of iPlayer's version). Still, i think he was working with Supercard from the beginning, when they were still using iPlayer as a kind of "DSTWO public prototype".