componets up front when ever possible. 5.25 vs 6.5 in the midbass range, when powered ff the HU power will be minimal difference.
Personally I would retain the OE rears and invest as much budget as possible in the front. 6.5+ tweeter would be ideal. then MAYBE a rear upgrade to a moderate coax.
The key is looking at Efficiency ratings. You want the highest "dB per watt/Meter" out put. if you can find 6.5 components @ the 91-92dB range off HU power is best of a not-perfect situation.
also, since you have the OE Sub it will do an admirable job at supplemnting the mid-bass and upper bass regions.
best of luck.
~Rob