Hit and miss by my experiences...
They want stupid money to do ANYTHING to my 172, e.g. £160 extra for putting on a magnex or lowering (each!!) for the 9 months left on my policy...
However they only asked for about £70 in total extra from Geoff for his lowering, new wheels etc when he got a quote, and also Jas only paid a small amount extra forhis Magnex etc.
So maybe then are OK when applying with mods, but try to hammer you if you do anything during your policy....