I always experience instability when I enable Smart Reflex above standard frequency. Sooner or later....

Here I should mention that this does not mean that for your N900 the results will be the same! It depends on the individual Chips characteristics, even made on conveyor line they are different when it comes to over-clock!
May be because of that Nokia disable it by default in the second firmware, for insurance that it will not cause mass instability, even for not over-clocked device.

So do not look for what (negative like me) people say but try it, and you will find out!