Sorry for the delay in replying ... guess I haven't visited this thread in the last month. Here's the link:
http://www.solux.net/solux_spec_page.htm

Claimed candlepower for 17 degree beams (for comparison's sake) is 4303 CP for the 3500K, 2782 CP for the 4100K (the bulb I have) and 2105 CP for the 4700K. So the 4100K loses quite a bit of efficiency versus the 3500K, but still significantly better than the 4700K.

I did also buy a 10 degree 4700K bulb for comparison, and it puts out noticeably less overall light than the 3500K. Being that narrow, it is of course quite a bit brighter in the center -- as its 5796 CP rating would indicate.

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What's really interesting to me is the comparison of the 4100K bulb with the Philips MasterLine EnergyAdvantage, the supposed efficiency champ for MR16s. I have the 24 degree, 35W version of this bulb, which is rated for 4400 CP -- in other words, with a much brighter AND wider beam, this 800 lumen monster should blow the SoLux away.

And yet to the naked eye it doesn't seem that way. In the proverbial darkened living room test the SoLux appeared brighter. I'd been staring at light bulbs long enought to question my own judgment, so I had my wife come and ask which one she thought was brighter. After the inevitable eye-rolling was over she said, "well, isn't it obvious?", indicating the SoLux. And out on the street, there is no question in my mind that the SoLux is more noticeable to motorists.