I wouldn't say a huge impact unless you can get to the 7 star and 9 star+ stages, but they do have some impact even at 4. I think it's a good idea to invest in them some but not to go overboard. Kruize also mentioned guild chests, but most do not invest too heavily in those unless they can do decent and grand investments to compensate.
Is the power added a fixed power or a power that's affected by level? If a hero level 1 gains 9 stars, is the bonus better for a hero level 70 with 9 stars, or is it the same? I'm saying this because you mention a fixed increase in power (level 5 chip).
My intuition tells me that the power added by stars is greater as the heroes gain levels.