From my experience, enabling SmartReflex with core freq above 850Mhz instantly reboots the phone; at 805Mhz and below it works flawlessly; and at exactly 850Mhz it works, but I start to get artifacts on the screen and dmesg errors related to the graphics chip. I have a hunch that's because the DSP frequency goes up to 500Mhz at 850+, but is only at 430Mhz for 600-805 (and lower still for core freqs below that). I'm lucky with my N900 though (since it was replaced by Nokia Care) as its CPU is good enough to be stable whilst running the "drained" config, so of course YMMV.