I've spent some time thinking about this over the years and have been watching that ebay post, haven't done it though. I'd like the taller 1st gear for sure, but for a road car it'd be great to have a nice overdrive gear, these sets don't have that, 6th is shorter than our stock 5th--0.704 compared to 0.660 respectively.
So the stock 5th with the 4.05 final drive is almost the same as the Quaife 6th with a 3.820 final.
Here's how the ground speed works out with a 225/45/17 tire (24.97 in diameter)
edit: Reviewing my numbers these graphics are a 3.820 final drive for the Quaife compared to a stock 4.050 for a Viggen or 9-5, so it's not apples to apples exactly:
I've been thinking about maybe going to the 3.820 final drive with one of these, but then the taller 1st might get out of hand with an aggressive clutch... I think if someone's running a large frame turbo with a narrow power band it might make sense to lay out the cash to do it. You're still going to need 4 gears for a quarter mile too.
There was also a straight cut Quaife gear set that had different ratios, but I haven't seen one of those sets for sale anywhere in a few years.