Don
The springs are the only bit they might care about. Seats might be classed as "interior modifications" but best to ask, they might not care, the rest I wouldn't mention.
Can't remember which Trim Mazda you went with, but unless it is a "Sport" where the Leather Seats genuinely could have been ordered as an option, then they will need to be declared. (as they may make the car more desirable and potentially increase the risk of being stolen etc)
From what I can gather from a quick look at the brochure, all Mazda 3's have the same ride height as well, and couldn't see any optional "Sports Suspension", so the springs need declaring as well.
the only other mods is the DPF Gutting and the Remap.
Which they are aware of and its stated as "Exhaust Modifications"
I have declared everything that isn't an option on my car. It cost £10 for the Exhaust Mod.
A remap should be declared as "ECU Tuning" or "Tuning Box" and a % or bhp improvement e.g (+20% or +40bhp) or something along those lines - declaring it as Exhaust Modifications isn't good enough.