It looks like the sid version of the Debian build (aka 1.99.4-something) has a lot of patches that are interesting for gentoo to fix a lot of problems with the emulator. For example many rice bugs (memory corruption, deadlocks, ...) and nearly all core patches seem to be important for gentoo. Without them 1.5 crashes with high resolution textures or just hangs when using fpu operations in some roms. http://patch-tracker.debian.org/package/mupen64plus-video-z64 http://patch-tracker.debian.org/package/mupen64plus-video-rice http://patch-tracker.debian.org/package/mupen64plus-video-glide64 http://patch-tracker.debian.org/package/mupen64plus-video-arachnoid http://patch-tracker.debian.org/package/mupen64plus-ui-console http://patch-tracker.debian.org/package/mupen64plus-rsp-z64 http://patch-tracker.debian.org/package/mupen64plus-rsp-hle http://patch-tracker.debian.org/package/mupen64plus-core http://patch-tracker.debian.org/package/mupen64plus-input-sdl http://patch-tracker.debian.org/package/mupen64plus-audio-sdl
Please also check http://anonscm.debian.org/gitweb/?p=collab-maint/mupen64plus.git;a=tree;f=debian/patches;h=9aeb81de4563acbdac0ff80e9b85cad291f0376b;hb=4e1d40f8087790ab9a393aee5361828bc8fc107a as it also has patches for other crashes/bugs
mupen64plus-1.5 is gone now.