Hey Jarkko,
I have quite a bit of experience with this actually, and it seems that memory manufacturer's have finally decided that memory megahertz speed is more important than low latencies.
You can see this because newer types of memory have way higher latencies and also higher clock speeds. In other words, if you can run at 1000mhz with 5-5-5-15 timings that is great, I have 4-4-4-12 memory myself which is 800mhz but I run it at 950mhz with 5-5-5-15 myself. Will give you more bandwidth than lower frequency with tighter timings will.
I always use Sisoft Sandra to measure memory bandwidth when I play with this stuff. Definitely get more out of your memory by cranking up the frequency and keeping the timings loose so the memory won't crap out. Of course you still want your latencies to be as low as possible but if you have to sacrifice something, sacrifice latency and not frequency.
Also, make sure to have your memory run 1:1 instead of with a divider and try to keep it at 1T although that might be a problem when overclocking. I've had to sacrifice 1T myself I think.