448f/336r is kinda high unless you are going for a 2"+ drop and don’t want the car to move much at all.
I would guess that stock rates for a Civic is 100-150 lbs, Eibachs and other aftermarket springs are 150-200 lbs. I’m sure there is a HT sticky that answers that very question.
Seeing that Civics have Double Wishbone/Multilink suspension there is no reason to run that high on the street (i.e there is no performance advantage with street tires and when your suspension does move negative camber gain isn’t a bad thing). Those rates are getting into competitve auto-X and R-comp controll.
With AGX’s the rule of thumb for spring rates is “don’t go more than 150% stock or you’ll blow them”. But I am about 450% on the original GAB design and the still work so I dunno.
The thing that you can do is get the kit and if you don’t like it pick up a pair of 250 springs for like $50 used. Then move the 336 to the front, 250s in the rear.
As far as “is it a crappy eBay product” I have never understood how GroundControl gets away with selling nothing more than a threaded fastener for $399.