The reason why Dark Horse R, GT4 and hell maybe even a GT3 does not have a STB is because they come equipped with a roll cage that also ties to the front structure of the car. Adding a welded in roll cage significantly stiffens the chassis of the car more so than just tying in the two struts on...
That kind of negative rear camber is typical for track use but not street use. Depending on what brand and type of tire you will probably see excessive wear around 8-15k miles. Keep in mind also the tire manufactures don't have any regulations for their sizing. So for example a 315/30 for one...
The performance advantages of the DH really show up when you take it to the track. IMHO if you don't plan on ever going to a local HPDE then save your money and go with a GT PP.