Roadworthy shouldn't be a problem, koni and others make inserts for a number of cars and suspension mobs do it fairly often. Make sure you tell your insurance company of course though, like any mod. You cannot, at all, tell it's an insert other than a nut being visible on the base of the shock that isn't there originally either, so there's sfa chance a cop or whatever would know you were running aftermarket shocks.
Firstly, I'm not aware of the rating of the whiteline springs, maybe an email to whiteline would get you an answer. They're not super-stiff springs, but certainly stiff enough to make the body roll in fast corners (anything you should enounter on the road at least!) quite acceptable.
The koni inserts that I got are only rebound adjustable, not compression adjustable. AFAIK this is the same as the shocks in most adjustable coilover setups. Whiteline springs with the shocks at ~2/3 to 3/4 of adjustment gives a very damped ride, so stiffer springs definitely could be used if that's what you wanted.
The job of swapping the insert involved a bit too much work for me, so I got the local Fulcrum suspension mob to do it for me. I think the price was around the $1200 mark, I'll have to find the receipt and get you the price and part numbers so you can see what local places can do. They did need to rebuild the new shocks straight up though to reduce their length somewhat to keep the spring captured, as there are no U13-intended inserts so some had to be adapted. However they didn't touch my car, I brought the shocks in and picked them up.
Man that was a long post. Let me know if you still want the info as ~$1200 is around D2 prices (however, I've driven cars with D2 coilovers, and made my decision based on that
)