I declared my strut brace, as I generally declare everything.]
It made no difference to the insurance, since I had lots of other stuff done this would have been peanuts in comparison.
My understanding is that your insurance can be void for undeclared mods..
See a recent post by Sandy somewhere, where his insurance company tried to do exactly that to him after an accident, and with some success unfortunately..
BTW I got the lovely lightweight aluminium and not very expensive one from alfisiti.net (1Kg or so).
Much to my suprise it immediately made an obvious difference both for road and track. I really wasn't expecting that to be honest, some slightly reduced sidewall wear on extreme track use was the extent of my hopes.
However my tyre wear on the side walls has been significantly reduced, but also the car feels better on the road, less tramlining especially (more solid).
I had thought there might have ben risk of understeer since anyhting that stiffens the front generally does this, but I guess it helps with camber retention by reducing relative movement of the strut tops..
..and overall its too cheap not to try yourself