don't forget that the 1.5 kernel can't handle memory cards >4GB. I would think DAX would have taken care of that problem already if he could.
With that in mind... what makes any of you think it would actually use the extra RAM of the slim? The latest homebrew had to be updated to take advantage... it doesn't use the RAM simply because it's there. The only 2 reasons people care about 1.5 is for the IR port (something that simply isn't going to be fixed, unless some hardware company designes a USB IR blaster for PSP, which I wouldn't hold my breath for) and the second reason is all the homebrew that hasn't been updated!
None the less... this is still a very impressive feat. Even for someone who cracks every new firmware that comes out.