There are a few ways that you could answer this question.
First, IQ is defined with a mean of 100 and a standard deviation of 15. But this is a normative score and relative to a given target population typically defined by time, geography, and various inclusion criteria. In particular, IQ scores have increased over time (i.e., Flynn effect). So by this definition, the meaning of a 200 IQ has changed over time. That said, you could pick a date and place (e.g., 1900 United States or the year 2000 United States) to define as the normative sample and that would give you a normative metric to work with.
You then have the issue of conceptualizing the standard deviation of intelligence. In general, there is a reasonable basis for assuming that such psychological traits are roughly normally distributed. But presumably, quantifying this as the high end is more difficult.
More generally, conceptualizing and measuring intelligence at the very high end is challenging. Most normal measures of intelligence max out at about 3 standard deviations above the mean (i.e., around 145). They generally would not be very reliable at distinguishing a 145 from a 160 IQ person.
It is also not immediately clear whether just making intelligence tests harder is sufficient to validly measure intelligence at the very high end. But it is presumably the starting point. Nonetheless, these difficulties in measurement and definition make conceptualizing a theoretical maximum quite difficult.
Another approach to this question is to consider geniuses throughout history. E.g., Einstein, Leonhard Euler. You will sometimes see projected IQs for such figures. It seems likely that these people who achieved eminence in highly cognitively demanding fields had very high intelligence. But disentangling intelligence, domain specific abilities, devotion to their discipline, training, disposition, and good fortune is challenging. So in general, determining whether they had a 150 or 180 IQs or something else is difficult.
More generally, presumably the eugenics argument is that humans are modifiable through selective breeding (and related processes), genetic modification, and possibly technological and pharmaceutical augmentation. This presumably opens up a whole, and potentially scary, world of possibilities for cognitive enhancement.
Another perspective to this question comes from artificial intelligence research. Presumably, as such technologies evolve to replicate and exceed more of what we consider to be uniquely human functionality, we may be better able to judge what is the conceptual limit of intelligence of any entity (biological or otherwise).