This is something that I've read a bit about following the posts on the forums.
There is one thing that I notice never comes up and that is that Super Beetle front springs are progressive- so the spring rates that Superman posted at Superbeetlesonly.com is for the first inch. The second inch will be stiffer than the first.
This is really important for a car with such a light front end.
I don't know if Steve Carters springs are liner or progressive, but I would imagine that 150 lbs linear springs would be very stiff for a beetle. But then Steve also has a radiator up the front there to.
S1fter has gone for 200 lbs progressive springs, so this is the rating for complete compression, which is about 10 % stiffer than stock. He chose that after testing a stock spring (then carefully filed the info in the bin!!!). But from memory he said that the first inch was about 70lbs, the second was about 150 ish and complete compression was about 180lbs.
Hope this helps - rather than throwing a spanner in the works
Cheers
Jeremy