sorry if this is in the wrong forum (still new here), but i have some news. I have been working on a 1.5 downgrader for 2.71, and have made progress!
When I launch my eboot (its a modified version of 1.50 firmware eboot), it launches and shows the psp logo screen. It then reads the memory stick for a few seconds and then shows up saying "file corrupted". The thing is though, that if you look closely, under the message you can see the startup screen for the update (where it says "PSP Firmware Update: [Start]").
Here is my eboot, remember it DOES NOT work yet, but it actually goes into kernel mode and then shows an error message saying "file corrupted" (unlike previous loads on 2.71 where it didnt even go into kernel mode and shows a similar message stating "Game could not be started"). If the error screen was removed somehow (working on it) then, in theory, you would have a 2.71>1.50 downgrader...
You can try it and see for yourself what I mean. I will post a photo as soon as I find my camera :)
Put the eboot in ...PSP/GAME/UPDATE folder