Besides the DSi having superior CPU and memory, the wifi is also apparently faster as well. So the pages wouldn't really load any faster than the normal opera DS browser anyways. >_>
If you think a better browser can be made, try coding one yourself. As for porting an official browser, it's pretty ridiculous, as ridiculous as asking someone to port a game without a source code. You'd have to reverse engineer it somehow, which takes an extraordinary amount of effort and skill (i only know of one game ever fully disassembled, recoded, and ported to another system by an amateur- Sonic 1). Then you have the trouble of DSi not being hacked so you can't even access the browser files to disassemble anyways... And again, it's like trying to put a game like Mario 64 on the GBA. What's the use when it will run horribly or not at all?