Read the fine print. Unless it specifically says that such accessories void the warrantee then it would not. Of course the dealer is the first step of any of that and they might say anything on the face of the earth (whether true or not.) Who knows. If you are really concerned about the answer, get hold of the regional Kubota rep and ask. Bro-tek is not exactly unbiased but they probably have come across this before and could give you an educated answer too.
Meanwhile, I assume you have already heard that the spacers can interfere with your belly mower depending on how long the spacers are, etc. My personal opinion is that the wider the rear tires are apart the more stable the tractor on slopes of noticeable steepness. Does not seem like 1.5" would be worth it to me but maybe it would. Others with that experience will hopefully comment for you.