Most people with 15s run 195/45.
And for other ways of lowering people have adapted corsa coilovers, polo coilovers and furyautoworks make Micra specific coilovers too. Also you can buy lowering springs but I don't reckon it's low enough for what you want
Sent from my Nexus 6 using Micra...
Tbh I haven't heard anything negative about them, might be worth trying a different set of plugs. Also have you checked for a vacuum leak? Even an audio recording would be good to diagnose it
Sent from my Nexus 6 using Micra Sports Club mobile app