No they dont mess the speedo cos most likely you will get bigger wheels with lower profile tyres, therefore your rolling radius will be almost the same as before.
Renault insurance, phone them before evry modification you do. Most will say its fine, mines lowered 60mm, 17s, colour coded, ice. etc. They cover £1k ice and up to £1k of styling mods. Obvious consideration is lowering bigger wheels etc will invalidate any warranty claims you try to make against steering or suspension problems.
Good luck enjoy.