I have found many threads on this but little conclusive, or generally accepted answers. If anyone finds one with a solid answer, please post a link.
It isn't really a 'it can make however much you want' (specially if you want realistic reliability), but at some point something fundamental will break, and that is the limit of the engine. Which is why the question gets asked. If you are going to sink money into an engine, 'what is the limit?'. Then you know if it is worth putting the money in to this engine, or if you should swap it for something that goes further. ....or perhaps decide it makes more sense to spend 1/10th of the money for 80% of the absolute max output.