The metal HG is thinner, not thicker, so it raises the compression ratio slightly. Worthwhile on a stock bottom end. If you want to do something different to the norm, K04, K03 and T25 have all been done! The K03 is popular for a reason, as it makes for a very nice car.
If you hook up a K03's watercooling to the Polo system it can get a bit warm if you're holding it flat out on a dyno (or 'private road') for any significant length of time, but on track it wasn't a problem on Yoof's. We got Robin's up to 110degC on the dyno at one point, but that was repeatedly getting up to and holding ~130mph road speed with a fan only simulating 40mph airflow.
Ideally a bigger radiator is needed. It's not just down to the turbo though, you're now making say 190bhp with a cooling system spec'd to deal with 113bhp - so there's a lot more waste heat to deal with!
With a purely oil cooled (and lubricated) turbo you're dumping the heat into your oil system, so you'd need a massive oil cooler instead.
I've not personally tried a K04, but given their price (for a genuine one, not an eBay Chinese 'K04' K03-copy with stronger actuator spring) a GT25R is an alternative - one that we're going to trial in Yoof's car.
