I have one, its an ex Motorolla cup/ grand am car. I'd agree with the spring rates recommended. Odly, I don't know mine, it was a pro car and it seems very good already. The car stiffness seems comparable in stiffness to a CRX and Integra R though I've not measured it. They are neat cars, more stable than a CRX, more tossable than the teg. Rev's to 8200 all day.
If you are thinking of building one, let me know and once spring comes around, I can do photo's of the cage, Also, my fabricator here in LaCrosse does excellent cage work for a lot less than you would get in bigger cities.
Ps. The cage rear braces on mine come from about 3/4 of the way up the roll hoop, and they punch through the rear firewall below the window and cross and are tied into the trunk floor. There is also a petty bar gong from the top of the cage to the passenger floor bar mounting.
Also I saw the comment on the hubs, and at least in my experience they appear plenty stout. Much bigger than CRX, In fact mine have lasted so long, I only replaced them because I broke a wheel stud.
Update - 2/27 - See marketplace, my Del Sol is for sale. I'm buying a Type R!