You think house and huis sound very different? I get just a smidge longer and flatter on the Dutch vowel, almost nothing... sort of Scottish sounding... and I have a pretty good ear. And I think I can say huis in a way that you'd approve of. Anyway, saying COWper for Kuiper comes way closer than any of the usual ways I hear it... because it's not spelled Kijper or Kuuper
Yeah, they sound very different to me. It's not just the length, there is a tonal difference and a difference in the rounding of the mouth as well. But it's true that when my English speaking friends pronounce the "ui", it does sound a lot more like the "ow" sound.
Here's the two diphthongs in graphical form, where ʌu is the IPA symbol for "ou"/"au" and œy the symbol for "ui":
The vertical and horizontal axes denote the first and second formants respectively. These are the resonance frequencies of the vocal tract which make up the first two frequency components of the sound - not counting the f0, which is the pitch of the voice, or fundamental frequency. Note that by convention, higher frequencies are usually plotted towards the left/bottom of the graph and lower frequencies to the right/top, so the axes are "reversed", as it were, from what you might normally expect. The arrows indicate how the sound changes from the onset of the vowel to the end. As you can see, they're in very different parts of the vowel space.
This all reminds me of an anecdote a phonetics professor I worked with had about some Japanese people who came to visit. They asked him whether "drama" should be pronounced "durama" or "dorama". He responded that it was neither, just "drama". They then nodded their heads and were satisfied to note that it was "durama" after all. Their brains literally were not wired to be able to hear the "dr" sound the way a native speaker could. They filled in the "u" because in Japanese, it is very uncommon for there to be two consonants in a row. They literally
could not perceive the difference. Supposedly you can learn to discriminate the sounds of another language to some extent, but it's quite difficult once you are no longer a kid.