Assuming an MCS had the following mods:

Alta CAI
Miltek Catback
15% pulley

will adding jcw injectors give further performance increases alone or do they need to be paired with a new cam?

Just adding the JCW injectors will not give you further performance unless you get a tune. Now if you throw in a Cam like the Nitrostick 1 +JCW injectors+ tune your Mini will be a totally new beast. But to answer your original question the JCW injectors will give you NO noticeable performance gains without a tune.