I know that Lightsmark is slow. Is any other driver faster? What about Intel? I remember when Lightsmark was released and by that time it was a damn demanding benchmark, I tried to run it on R500, it was slow even with fglrx IIRC.
Beiruty, if you make a compeling argument about why r300g+R520 should be fast in Lightsmark, we may have a look at what's slow there and fix it. But see if fglrx is faster first.
Also I had a discussion with the author of Lightsmark (he's from the same country as me, and presented some alpha version of Lightsmark for a local game development community here) a few years ago and IIRC indirect lighting (i.e. radiosity) in his benchmark was computed on the CPU, while direct lighting and shadows were computed on the GPU, so GPU performance might not matter that much if you're CPU-limited.
I douwnloaded and installed lightsmark 2008 V2 on winxp. It run and it sarted great between 30 fps and 60 fps with Audio Music. However, less than a minute screen corruption started showing and crashed the video card.
I guess AMD OpenGL on winxp are not that great either! But I saw amazing fps!