My system is (2) SSi 15's pushed by a 2500d orion amp if i buy a HO 250amps Alt. is there a need for a cap? My quess is no but if im wrong tell me plz.
You are right, you don't need a cap. Actually no one needs a cap. Only other thing besides alternator that people should consider is an extra battery, but you don't need that with that alternator
A cap could help, yes. In this situation, a capacitors function is to make sure the bass hits accurately and relieve a voltage drop until the alternator responds. The alternator can only respond so fast, but a capacitor supplies quick power. As long as you have the HO alternator then a capacitor would just act as a buffer in the system, therefore allowing the voltage not to fluctuate.
If you have a HO Alt, there really is no reason to have a cap. The difference it makes is so minute it's not worth the money IMO. The gain is negligible at most.