Well, the time needed to setup thumb target in scratchbox is a about an hour(at most, along with the time needed to download the toolchain). I don't know if you compile in SB or cross-compile, but if latter, building with thumb should be matter of passing -mthumb to the compiler (assuming you use gcc and binutils >= 2.19). Though if there is some assembler trickery in javascript engine, the above would not work.

Anyway, nothing to be done here, opera is closed source after all.

EDIT:

On the other hand I am impressed with how opera scores on sunspider:

CSSU-thumb, max CPU@600 (no overclock) - 5583.1 ms