The Stock 08 WRX springs/struts are probably the softest on any year WRX/STi by a pretty substantial margin. They were gone in 09. No, they won't handle RCE black springs (probably not enough to handle 09 WRX springs even). STi struts really aren't enough for RCE blacks (but will work supposedly). If you really want to improve your suspension, you're going to need to do the struts with the springs. RCE blacks pair best with aftermarket struts like d-specs.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD