Try comparing the following registers between fglrx and radeon:

0x98FC
0x98F8
0x8950
0x98F4
0x3F88
0x9B7C
0x3F90
0x9148
0x3F94
0x914C
0x8954
0x2004
0x2008
0x2768
0x8B24
0xA008
0xA020
0xA02C
0x9100
0x913C
0x960c
0x9610
0x88C4

You can use radeonreg (as root):
./radeonreg regmatch 0x98FC

See if changing any of them to what fglrx programs before starting X with
tiling disabled.