In answer to the last question on this thread (from Reflected Soul) the custom firmware's primary improvement is proper mapping of the L & R buttons for GBA games. You may or may not also include nifty pixel art backgrounds and icons in the category of improvements.
Now for the bad news. lsd_blottr seems to have given up on creating future custom firmware for Letcool type devices. There is always the unlikely possibility that someone is impersonating him but here is what he has (presumeably) posted on sunnap's blog:
i SUDDENLY install linux and stoppd any deals with this console... After my last custom firmware i don't see anything that i can do wit this konsole.
Fortunately, these comments were in response to the great work that sunnap is doing to bring Linux to Letcool and other SPMP8000 devices. He has created a new firmware flashing tool called Sunburn which is more compatible with the Letcool. Hopefully this is an early step on the way to running Linux on SPMP8000 devices. You can read more about it on sunnap's blog:http://sunnap.blogspot.com/2011/06/sunburn-firmware-flashing-tool-for.html#comments