8.7 kW in an energy-saving regulation house, do you want to suffocate? We have 5 and that is already more than enough.
What do you want to say with that? The kW specifications on the fireplace only indicate how much you *could* fire, but not how much you *have to*! You can also only put a few logs on a fireplace designed for 8.7 kW: small fire and short duration = little heat.
If the chimney sweep has approved it (for us, the notice for the shell construction acceptance, for example, states 7-8 kW), then you can definitely make full use of that, a larger fireplace always looks better than a too small one. (My opinion) How little you actually fire then luckily remains up to you.