If it is 6.0mm stick to having the 8,5 kw that is just about ok. if you go for the 10.5 you need to have a 10mm tw/e cable installed a very costly job that will also have to be certified to local council which means an electrician who is able to self certify works etc, keep it simple 8.5 is fine for a shower anyway. Ray
PS what else you need to consider is the cable and plumbing entries on the shower do the match exactly? they should do with it being the same make, but just check it out, sometimes they change positions of them.