On a 1341cc with a big valve head you're absolutely fine to push a K03 to 16-17psi before it starts getting remotely inefficient - don't know where you got 10psi from! Suggest you do the maths again.
The limiting factor on the turbo is usually the max rpm, which is proportional to flow - not boost. So for the same turbo 21psi into a lawnmower engine is lots easier than 15psi into a 5.7l V8. Bear in mind that many 20v tuners will run the K03 at 19psi or more on an 1800cc 20v engine that'll flow more air than an 8v Polo small block could ever dream of - so even pushing it to 21psi or so on the Polo lump isn't going to be stressing it.
High inlet temps are usually a sign that you're hammering your turbo hard. However, even running 20psi on a 25degC ambient day I've not seen inlet temps much beyond 35degC thanks to a decent intercooler - so I'm not worrying. Crudely using a type K thermocouple and meter on Yoof's setup indicated his was pretty similar.
Yoof has only used the gas on the drag strip and dyno, it's not in for regular use as his gas bottle is strapped to a spare passenger seat base... So when he says it spools quickly, that's without the nitrous. The gas is whacked in on the strip around 3k, just as the turbo's spooled. Not much point below then as it's wasted in wheelspin anyway. The gas is what makes the car break 200bhp though, I'd say it's around 190bhp without it.
It's worth mentioning camshafts at this point too - a 3F cam with a K03 means you can have 15psi of boost at around 1500rpm on a 1341cc lump. Which sounds great, 'til it completely runs out of puff at 5k. That's why the PSD turbo cams were a fair bit lairer than a 3F cam, which means the good stuff doesn't start to happen 'til around 2.5-3k, but you get a more useable powerband to 6-6.5k.
If you want a driveable car to use on track and road then go for a K03 if you've got a 1341cc bottom end or smaller. If you're after dyno figures or a track car stick a K04 on instead. You'll make higher numbers, but it'll shift your powerband up some more. You could swap to a tamer cam to move the powerband down - but you'd likely loose the higher peak bhp figure in doing so. In short, you can't have your cake and eat it otherwise we'd all be running Garrett GT40Rs.
I agree with there being no such thing as a K03s - Borg Warner will tell you that all day long. Aside from minor revisions in the wheel sizes/design, the only significant difference is actuator spring strength as already mentioned.
You're right in saying that a K04-015 should fit straight on, Passat power figures seem to suggest 200bhp for a K03 and 230-240bhp with a K04. I'm not sure that you can directly translate those to a Polo setup though. There are a few K03'd cars out there now, and many people are claiming silly power figures - but those I know that've been on the dyno have made no more than 190bhp without nitrous.
When I get some more time (when the Polo's not 100 miles away from me) I intend to finish doing my high boost maps. I've made 133bhp @ 7psi and 175bhp @ 15psi at Emerald, and am working on a 21psi map which should see 190bhp or so as the butt dyno suggests it's a lot pokier than at 15psi. I'd like to try to push things further than 21psi, just to know at which point it becomes pointless running any more boost from a K03.