Q: Can this mod work on other Android devices? Would we see a performance boost on them as well? If not why is it limited to the Galaxy S?
A: SGS has very very good hardware, but it has some parts of it's hardware poorly implemented. The filesystem that samsung chose to use is custom-built using FAT32 as a base, RFS. It has a lot of the problems that FAT32 has, and should have been left back in the 90s, or even the 80s.
One of the big issues with it is how it handles multiple requests - it blocks. It blocks everything. When your mail app wants to read the mail you just tried to view, but your twitter app is busy writing a new tweat it just received, your mail app is forced to wait.
This is bad, but it could be worse! And it is... your twitter app didn't just get one tweat, it got 50 tweaks. It is busy writing the tweats one by one to the filesystem. This would be fine, since all modern filesystems will buffer writes, so instead of writing each one at a time, they will batch them together and write it as a big chunk. Uh oh - RFS does no buffering at all! After each write, it will also write an update to the grafted-on journal system. Guess what happened to your mail you were trying to view while all this happened? It 'lagged' and you got a black screen for half a second, before the mail popped into view.
Luckily the hardware on the device is so good that you usually don't even notice the problem until you have a lot of apps running, all writing their updates when you unlock the phone.
This is mostly speculation based on experiments done on RFS -- RFS is closed source, and we have no idea if the problems are just badly set settings (such as a block size that is too small), coding bugs in the implementation, or if RFS is just really that badly designed.
This fix just grafts a buffer on top of the RFS filesystem, using a very very simple and fast filesystem, EXT2. It fixes most of the issues by writing to RFS as seldom as possible.
So no, this fix won't fix other devices, since they're already running quite close to maximum speed. The SGS at stock is running nowhere near maximum speed, and this lag fix takes it a bit closer. You could probably speed up other devices by tweaking the filesystem settings to give them a big buffer or similar, but it isn't really needed. (I haven't actually tried to put an EXT2 onto any other Android phone, as I don't have any other Android phone, so this is just speculation.)