1.5 kernel cannot see more than 4 gigs. Period. Try placing a few movies/umds (4 gigs worth) on your 8 gig then add some homebrew emulators (1.5 kernel versions, not ones made for OE firmwares) afterwards, such as snes/nes/daedalus... it just won't work most of the time. some of them actually load but when you try to start a rom, it will crash the PSP.
of course anything that works on the slim can use an 8gig stick because anything working on the slim is using kernel 3.6+ (now 3.71)