... 20mm spacers with 215 tires (that may be closer to 220 actually) assuming it will be on the stock wheels, if the car ever gets lowered you are screwed, the back will rub, there is no way around it ! I have 18x7.5" ET48 wheels, 205 tires that are more ....215 say, 11mm spacers, 30mm down, the induced camber doesn't make it for the extra poke, and we will have to trim the inside lip of the arch, or find smaller spacers, if not "no" spacers for the back.
Without lowering, or a lot less, like 20mm, sure, but it probably will look ... funky.
In my case, the front is absolutely flush with the extended arch of the JCW, (wheel + spacer is an ET 37 with the extra poke from the wider wheel) the rim exits the fender a little but in person, it just looks perfect. The back really is where we found things to be surprisingly harder to settle.
Also, with big spacers, bearings take a lot more in the teeth, it is a mechanical effect. Sure it won't destroy them right away, but if you put very sticky tires and extra large spacers you will inevitably induce a lot more forces on those bearings. Lighter wheels, in the other hand, can lighten the burden a little.
In the case of the JCW, it is a tricky equation as you know, no matter what ! ... but there are good proven setups out there. Only if you want to be picky about the wheels, it can get frustrating.