I got so many negative reviews about spacers that I decided not worth it. Usually "sifus" recommend smaller offset wheels instead of spacers, which is what i did. If you want to get 15mm spacers, do you plan to roll the fenders? That is a big offset. If currently you are using +40et rims, then a 15mm spacers is equivalent to +25et rims. The spacers aren't cheap either so it's worth considering.
Hubcentric cones/rings and spacers are usually used to snug fit a larger diameter hub openings (or spigot) rim into a smaller wheel hub, meaning there will be a gap. You don't need it unless you are planning to fit in some JDM or e39 rims. Having said that, even people using larger diameter spigot rims don't install cones/rings and get away with it (lug-centric fit). Nasib-lah... reaching the destination may take long but journey is the fun part.