Well it will make it more expensive!
You don't have to tell them, but you do run the risk that if you have an accident they will use it as an excuse to void your insurance and leave you with a huge bill.
I have Eibach springs on my car, but they are the official Mazda springs so don't really qualify as a modification I think.
It's up to you, but you do run the risk of voiding your insurance, also be prepared for them to slap a chunk on top.
Give us some pics too, would be keen to see your car, as I have a 1.5 Sport also, 59 plate
