Got a call from Indy at IS Motor Racing the next morning with an apology on not calling me back. He offered to install the springs for me the same day. Now that's great customer service! Recognising the problem and offering solutions rather than excuses.
So I dropped off my car and a few hours later returned to pick up something that had a lot more attitude. Don't get me wrong, the STi springs are great, but there was still quite a bit of body roll on the hard corners and it just never quite felt settled. Plus it just didn't look quite right at that ride height for my own taste.
Now the car sits more aggressively, the ride is firmer and more controlled, and cornering has much reduced body roll. On smooth surfaces and on minor bumps and cracks it actually feels a bit better than the STi. The Tein's would absorb it a bit better, whereas the STi would jostle the car a bit. Ride comfort is same as the STi's, which is quite an accomplishment.
On sharp bumps the difference leans towards the negative. The impact hits a little harder and the car shudders a bit more than the STi's. But I guess that's the trade off. Overall though, I think the positives outweigh the negatives.
In any case, I'm still in the 'honeymoon' period. I found that after 2 - 3 months, when the springs have well and truly settled, the ride tends to get a bit rough, so we'll see how the Teins fare.